docs: cross-reference json_view and json_document for 3.13.0 (#5892)

* docs: cross-reference json_view and json_document for 3.13.0

Document the new zero-copy document/view API across the docs: add a
"Documents and views" section to the basic_json API hub linking to all
specializations, point parsing/conversion/modifying pages to the view-based
alternatives, add document/view sections to feature pages, update the
landing page to mention json_view.hpp/json_literals.hpp, and cross-link
debugging/exceptions.

Also document wide-string inputs (and ill-formed UTF-16/32 rejection),
add parse/accept examples for wide strings, fix related doc gaps, and
align features/parsing and FAQ with the added support.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

* docs: fix article before std::istream in FAQ

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

* docs: clarify document inputs, decoding, get<T>() and editing

Address review comments: say which inputs are borrowed and that a wide
string pointer is an extra input, which values are decoded during parsing,
that get<T>() reads only built-in types directly, all forms of set() and
that the document's insert() is array-only, and how to store wide strings.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

* docs: integers are converted while parsing a document

Also say "They accept every input" in basic_json_document::parse.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

---------

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

docs/mkdocs/docs/features/macros.md

@@ -287,6 +287,23 @@ external dependency and is not defined by default.
 
 See [full documentation of `JSON_USE_SIMDUTF`](../api/macros/json_use_simdutf.md).
 
+## `JSON_VIEW_NO_SIMD`
+
+When defined, the parser of [`basic_json_document`](../api/basic_json_document/index.md)
+(`<nlohmann/json_view.hpp>`) uses only portable C++ to scan strings, instead of vector instructions (SSE2 on x86-64,
+NEON on AArch64). The same input is accepted or rejected either way; only the speed differs. It is not defined by
+default.
+
+See [full documentation of `JSON_VIEW_NO_SIMD`](../api/macros/json_view_no_simd.md).
+
+## `JSON_VIEW_USE_SSSE3`
+
+When defined on x86-64, the parser of [`basic_json_document`](../api/basic_json_document/index.md)
+(`<nlohmann/json_view.hpp>`) validates non-ASCII text in strings with SSSE3 without asking the CPU first. It is not
+defined by default.
+
+See [full documentation of `JSON_VIEW_USE_SSSE3`](../api/macros/json_view_use_ssse3.md).
+
 ## `NLOHMANN_DEFINE_TYPE_*(...)`, `NLOHMANN_DEFINE_DERIVED_TYPE_*(...)`
 
 The library defines 12 macros to simplify the serialization/deserialization of types. See the page on