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/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"