Split the json_view feature page into sub-pages (#6018)

* Split the json_view feature page into sub-pages

The single features/json_view.md page grew to ~600 lines covering eight
topics. Split it into features/json_view/ with an overview page and
pages for ownership and lifetime, differences from basic_json, reading
values, generic code, editing, and images.

The new differences page collects what was spread over several sections
("What is the same", "What is different", "Duplicate keys", and notes in
the conversion and serialization sections) under one summary table, and
adds the parts of the basic_json interface a view does not have. The
other pages now link to it instead of restating the rules.

Internal links across the documentation point to the new pages.

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

* Docs: fix stale comments and trim repeated text in json_view pages

- fig-jsonview.js/.css: point the header comments at the new pages
- reading.md: views' strings are valid as long as the document (and a
  borrowed source text), as lifetime.md says
- reading.md, generic_code.md: link to differences.md instead of
  restating the member-order and repeated-key rules
- differences.md: drop the third mention of ordering comparisons, name
  rbegin()/rend() in the iterator row
- exceptions.md: link editable documents to editing.md

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

---------

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

docs/mkdocs/docs/api/basic_json_view/get.md

@@ -39,14 +39,14 @@ as [`BasicJsonType::parse()`](../basic_json/parse.md) does), as well as the type
 accept views, e.g. with [`is_json_view`](../is_json_view.md). A `from_json()` that also accepts types that are no JSON
 values at all, such as the unconstrained `#!cpp template<typename BasicJsonType> void from_json(const BasicJsonType&,
 T&)` of [arbitrary type conversions](../../features/arbitrary_types.md), is not called with a view: it may use
-`BasicJsonType` functions a view does not have. See [Writing generic code](../../features/json_view.md#writing-generic-code).
+`BasicJsonType` functions a view does not have. See [Writing generic code](../../features/json_view/generic_code.md).
 
 !!! warning "Duplicate keys"
 
     Such a `from_json()` looks members up in the view, where a repeated key finds the **first** member, while
     `#!cpp materialize().get<T>()` sees the **last** one, as [`BasicJsonType::parse()`](../basic_json/parse.md)
     keeps it: for `#!cpp {"x":1,"x":2}`, `#!cpp get<point>()` reads `#!cpp x == 1`. See
-    [Duplicate keys](../../features/json_view.md#duplicate-keys).
+    [Duplicate keys](../../features/json_view/differences.md#duplicate-keys).
 
 Every other `T` — user types whose `from_json()` is written for `BasicJsonType` only, types without a default
 constructor, `#!cpp std::tuple`s holding references, ... — is converted by `#!cpp materialize().get<T>()`: the subtree is built into a real `BasicJsonType`
@@ -152,7 +152,7 @@ instead: its path is relative to the root of the view's subtree, because `materi
 - [`number_token`](number_token.md) - a number's token text, without a copy
 - [`materialize`](materialize.md) - build the `BasicJsonType` value of this subtree
 - [`BasicJsonType::get`](../basic_json/get.md) - the corresponding function of `basic_json`
-- [Zero-copy JSON views](../../features/json_view.md#getting-values-out-without-copying) - which conversions avoid building a basic_json value
+- [Zero-copy JSON views](../../features/json_view/reading.md#getting-values-out-without-copying) - which conversions avoid building a basic_json value
 
 ## Version history