docs: new design for the documentation site (#5934)

* docs: new design for the documentation site

Apply the brand to the Material for MkDocs site: Inter and JetBrains Mono
served by the site (upright and real italic), brand colours, blue header band
with the syntax pattern, wordmark in the header, new home page (hero, cards,
animated feature tour), 404 page, polish for tabs, search, footer, focus
rings, tables and code blocks, and theme features (code copy, previous/next
footer, TOC follow, search suggestions).

Draw the number railroad diagram, the parser callback events and the iterator
illustrations as inline SVGs that follow the light and dark scheme. The
generator scripts are in docs/mkdocs/scripts/figures; the feature slideshow
(json-light.svg, json-dark.svg, replacing json.gif on the home page) is built
by docs/mkdocs/scripts/slideshow. Decision nodes in five flowcharts become
hexagons, which need far less room.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

* docs: add figures, table captions and a macro chooser

- Figures with numbered captions: SAX stepper, json_view layout and edit
  stepper, exception map, JSON pointer and patch walkers, QA pipeline,
  scaling and binary size charts, roadmap lanes, release timeline,
  integration decision tree and namespace builder
- Table captions (nj-table-caption) on 9 pages
- Compile times: modules and PCH charts, filesystem section, re-measured
  Figure 1, links between features/modules.md and compile_times.md
- Interactive macro chooser for features/arbitrary_types.md
- Link check: exclude URLs that answer the plugin with 400/401/419 or are
  dead (customers.md)

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

* docs: macro chooser generates adl_serializer for non-default-constructible types

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

* docs: cross-link concept pages and API pages in See also sections

Add See also links in both directions between the concept pages (features,
home, integration) and the API pages, write API links in code font, give every
entry a comment after a dash, and show the callback event figure on the parser
callbacks page.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

* docs: address review comments on the new design

- Home: add a "Latest release" button
- Sponsors: update the list and group it by tier
- Features: list all feature pages on the index page
- Releases: add highlights per release, link the 4.0 roadmap, add the first commit to the timeline
- Exceptions: prefix all example messages with the exception name
- API: no code font for format names in the See also links of the to_/from_ pages
- Merge Patch: show the document before, the patch, and the result
- Replace "--" with typographic dashes, mark external links with an icon, center Mermaid diagrams

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

---------

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

docs/mkdocs/docs/features/conversions.md

@@ -177,7 +177,7 @@ which forces the explicit `get` form and can catch unintended conversions at com
     `std::optional<T>` from delegating to `get<T>()`, to avoid a constructor ambiguity with
     `std::optional<T>`'s own converting constructor from `basic_json`. As a result,
     `static_cast<std::optional<T>>(json_value)` goes through `std::optional<T>`'s own converting
-    constructor rather than through `get<std::optional<T>>()`, which can behave differently -- for example,
+    constructor rather than through `get<std::optional<T>>()`, which can behave differently — for example,
     with a custom `adl_serializer<std::optional<T>>` specialization. Prefer `get<std::optional<T>>()`/`get_to()`
     over `static_cast` for optional types.
 
@@ -193,7 +193,7 @@ which forces the explicit `get` form and can catch unintended conversions at com
     - The JSON array has **too many** elements: the surplus is **silently discarded**, and no exception is
       thrown.
     - The JSON array has **too few** elements: `at` throws
-      [`out_of_range.401`](../home/exceptions.md#jsonexceptionout_of_range401) for the first missing index --
+      [`out_of_range.401`](../home/exceptions.md#jsonexceptionout_of_range401) for the first missing index —
       an out-of-range error, not a [`type_error`](../home/exceptions.md#type-errors), even though the cause
       is a shape mismatch.
 
@@ -211,7 +211,7 @@ which forces the explicit `get` form and can catch unintended conversions at com
 
 ## Omitting a field when serializing `std::optional`
 
-By default, `to_json` for `std::optional<T>` writes either the value or `#!json null` -- there is no built-in way
+By default, `to_json` for `std::optional<T>` writes either the value or `#!json null` — there is no built-in way
 to make a field disappear from the serialized object entirely when the `std::optional` is `std::nullopt`. Because
 a specialization of `adl_serializer<std::optional<T>>` only controls how the *value* is converted (it cannot
 prevent the containing object's `to_json` from inserting the key in the first place), omission has to be
@@ -243,7 +243,7 @@ json j = numbers;   // [1,2,3]
 !!! info "Constructing from a C++20 range view"
 
     A `json` array can also be constructed directly from a C++20 range view (`std::ranges::view`), such as the result
-    of `std::views::filter` or `std::views::transform` -- no intermediate container is needed:
+    of `std::views::filter` or `std::views::transform` — no intermediate container is needed:
 
     ```cpp
     std::vector<int> nums{1, 2, 37, 42, 21};
@@ -266,7 +266,12 @@ The conversions above are built in for standard types. To make the same syntax w
 
 - [`get`](../api/basic_json/get.md) - get a copy converted to a given type
 - [`get_to`](../api/basic_json/get_to.md) - convert into an existing variable
-- [`get_ref`](../api/basic_json/get_ref.md) / [`get_ptr`](../api/basic_json/get_ptr.md) - access the stored value without copying
+- [`get_ref`](../api/basic_json/get_ref.md) - / [`get_ptr`](../api/basic_json/get_ptr.md) - access the stored value without copying
 - [Arbitrary types conversions](arbitrary_types.md) - support your own types
 - [`JSON_USE_IMPLICIT_CONVERSIONS`](../api/macros/json_use_implicit_conversions.md) - toggle implicit conversions
 - [Zero-copy JSON views](json_view.md) - get values out of a document or a view
+- [`adl_serializer`](../api/adl_serializer/index.md) - the default serializer behind the conversions
+- [`type`](../api/basic_json/type.md) - query the type of a value
+- [`JSON_DISABLE_TUPLE_REFERENCE_CONVERSION`](../api/macros/json_disable_tuple_reference_conversion.md) - toggle conversion to tuples of references
+- [`JSON_HAS_RANGE_VIEW_CONVERSION`](../api/macros/json_has_range_view_conversion.md) - toggle conversion from ranges
+