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/api/basic_json/accept.md

@@ -23,6 +23,9 @@ Checks whether the input is valid JSON.
     respectively as UTF-8, UTF-16, and UTF-32. If `SentinelType` differs from `IteratorType`, it must be comparable to
     the iterator type with `operator!=`.
     
+    Ill-formed UTF-16 and UTF-32 sequences are rejected: a lone surrogate, a high surrogate that is not followed by its
+    low surrogate, and a code point above `U+10FFFF` all make this function return `false`.
+
 Unlike the [`parse()`](parse.md) function, this function neither throws an exception in case of invalid JSON input
 (i.e., a parse error) nor creates diagnostic information.
 
@@ -35,9 +38,12 @@ Unlike the [`parse()`](parse.md) function, this function neither throws an excep
     - a `#!c FILE` pointer (throws if null)
     - a C-style array of characters
     - a pointer to a null-terminated string of single byte characters (throws if null)
-    - a `std::string`
+    - a `std::string` or a `std::string_view` (C++17)
     - a container `obj` for which `begin(obj)` and `end(obj)` produce a valid pair of iterators
       (as found via ADL or member functions, with semantics compatible to `std::begin` and `std::end`)
+    - wide characters (`wchar_t`, `char16_t`, `char32_t`), which are converted to UTF-8: a wide string object
+      (`std::wstring`, `std::u16string`, `std::u32string`) or an array (e.g. a wide string literal); a pointer to a
+      null-terminated wide string (e.g. `std::u16string::c_str()`) is not supported
 
 `IteratorType`
 :   a compatible iterator type, for instance.
@@ -125,6 +131,21 @@ outside of a string, invalid) byte; see the [FAQ entry](../../home/faq.md#nul-by
     --8<-- "examples/accept__iterator_pair.output"
     ```
 
+??? example "Example: (3) reading from UTF-16 strings"
+
+    The encoding is deduced from the character type of the input. String literals like `u"..."`, `U"..."`, and
+    `L"..."` work as well.
+
+    ```cpp
+    --8<-- "examples/accept__wide_string.cpp"
+    ```
+
+    Output:
+
+    ```json
+    --8<-- "examples/accept__wide_string.output"
+    ```
+
 ## See also
 
 - [parse](parse.md) - deserialize from a compatible input
@@ -151,6 +172,9 @@ outside of a string, invalid) byte; see the [FAQ entry](../../home/faq.md#nul-by
   ([`JSON_DISABLE_STREAM_BULK_SCAN`](../macros/json_disable_stream_bulk_scan.md)).
 - Parsing from a `FILE*` locks the file once instead of once per character in version 3.13.0
   ([`JSON_DISABLE_UNLOCKED_FILE_IO`](../macros/json_disable_unlocked_file_io.md)).
+- Ill-formed UTF-16 and UTF-32 input (a lone surrogate, a high surrogate that is not followed by its low surrogate, or
+  a code point above `U+10FFFF`) is rejected in version 3.13.0; before that, some such sequences were accepted or
+  ended the input silently.
 
 !!! warning "Deprecation"