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/css/fig-jsonview.css

@@ -1,4 +1,4 @@
-/* figures: jsonview (features/json_view.md; SVGs from scripts/figures/json_view_layout.py, stepper in js/fig-jsonview.js) */
+/* figures: jsonview (features/json_view/*.md; SVGs from scripts/figures/json_view_layout.py, stepper in js/fig-jsonview.js) */
 
 /* one colour per concept, the same in every figure: structure (object/array, views) blue, keys navy, strings green,
    numbers orange, decoded strings purple, owned edit storage teal, dangling/invalid red */