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/mkdocs/docs/api/macros/json_no_simd.md

@@ -80,7 +80,7 @@ By default, `#!cpp JSON_NO_SIMD` is not defined, and vector instructions are use
 - [Parsing: input sources and performance](../../features/parsing/index.md#input-sources-and-performance) - how input
   sources affect parsing performance
 - [Serialization](../../features/serialization.md) - pretty-printing, UTF-8 handling, and number output
-- [Zero-copy JSON views](../../features/json_view.md) - the zero-copy view, whose parser the macro also affects
+- [Zero-copy JSON views](../../features/json_view/index.md) - the zero-copy view, whose parser the macro also affects
 
 ## Version history