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/index.md

@@ -50,7 +50,7 @@ Besides `basic_json`, the library offers a second way to hold a parsed JSON text
 [`basic_json_document`](../basic_json_document/index.md), which owns or borrows the text and holds a flat index of the
 values in it, and a [`basic_json_view`](../basic_json_view/index.md), a read-only handle to one value of such a document.
 Both are defined in the header `<nlohmann/json_view.hpp>`; see
-[Zero-copy JSON views](../../features/json_view.md) for the concept and the trade-offs.
+[Zero-copy JSON views](../../features/json_view/index.md) for the concept and the trade-offs.
 
 - [**json_document**](../json_document.md) / [**ordered_json_document**](../ordered_json_document.md) - read-only
   documents of the `basic_json` specialization they belong to
@@ -363,7 +363,7 @@ Access to the JSON value
   specification of the JSON data interchange format
 - [`basic_json_document`](../basic_json_document/index.md) - / [`basic_json_view`](../basic_json_view/index.md) - parse a JSON
   text into a flat index and read it through a view
-- [Zero-copy JSON views](../../features/json_view.md) - when to use a document and a view instead of a `basic_json`
+- [Zero-copy JSON views](../../features/json_view/index.md) - when to use a document and a view instead of a `basic_json`
 - [Types](../../features/types/index.md) - how JSON values are stored and which types exist
 - [Converting values](../../features/conversions.md) - get values out of a JSON value and put values in
 - [Creating JSON values](../../features/creating_values.md) - all the ways to build a JSON value

docs/mkdocs/docs/api/basic_json/parse.md

@@ -124,7 +124,7 @@ outside of a string, invalid) byte; see the [FAQ entry](../../home/faq.md#nul-by
     [`basic_json_document::parse`](../basic_json_document/parse.md) accepts the same inputs and the same options, but
     keeps the result as a flat index of the parsed values instead of a tree of `basic_json` values. It is faster and uses
     less memory when only parts of the document are read, and it also accepts a pointer to a null-terminated wide string,
-    which `parse()` does not. See [Zero-copy JSON views](../../features/json_view.md).
+    which `parse()` does not. See [Zero-copy JSON views](../../features/json_view/index.md).
 
 !!! tip "Performance"