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/features/parsing/index.md

@@ -86,7 +86,7 @@ ahead: after parsing, it is positioned exactly as if every character had been re
 [`operator>>`](../../api/operator_gtgt.md) can still read several values from one stream (see also
 [JSON Lines](json_lines.md)).
 
-This does not hold for [`json_document`](../json_view.md): it reads a stream into a buffer it owns, so the stream saves
+This does not hold for [`json_document`](../json_view/index.md): it reads a stream into a buffer it owns, so the stream saves
 no memory, and reading the file into a `#!cpp std::string` yourself and borrowing it is about 6 to 12 % faster.
 
 Two macros switch these optimizations off without changing any result:
@@ -115,7 +115,7 @@ The library offers two parsing models, and a third, read-only alternative:
   reads the same JSON grammar but builds a flat, read-only index of the values instead of a `basic_json` tree. Strings and
   floating-point numbers are read from the text on demand, and only the parts you
   [`materialize()`](../../api/basic_json_view/materialize.md) become `basic_json` values. This avoids most of the work when you
-  need only a few values from a large or frequently parsed text; see [Zero-copy JSON views](../json_view.md). It is read-only, so use
+  need only a few values from a large or frequently parsed text; see [Zero-copy JSON views](../json_view/index.md). It is read-only, so use
   `parse` when you need to modify the value.
 
 You can influence a DOM parse without switching to the SAX interface by passing a
@@ -139,7 +139,7 @@ json_document doc = json_document::parse(R"({"happy": true, "pi": 3.141})");
 std::cout << doc.root()["happy"].get<bool>() << '\n';
 ```
 
-See [Zero-copy JSON views](../json_view.md) for the concept and the trade-offs.
+See [Zero-copy JSON views](../json_view/index.md) for the concept and the trade-offs.
 
 ## Exceptions
 
@@ -158,7 +158,7 @@ options.
 - [Large, streamed, and partial input](large_and_partial_input.md) - chunked, concatenated, and very large input
 - [parser callbacks](parser_callbacks.md) - influence the parsing by a callback function
 - [SAX interface](sax_interface.md) - implement a custom SAX handler
-- [Zero-copy JSON views](../json_view.md) - parse into a read-only document and read it through views
+- [Zero-copy JSON views](../json_view/index.md) - parse into a read-only document and read it through views
 - [parsing and exceptions](parse_exceptions.md) - control error handling
 - [parsing untrusted input](untrusted_input.md) - what to consider when parsing input from untrusted sources
 - [`operator>>`](../../api/operator_gtgt.md) - deserialize from stream