Document MessagePack ext types and fix related doc errors (#5928)

* Document MessagePack ext types and fix related doc errors

Add an "Extension types (ext, fixext)" section to the MessagePack page:
ext/fixext are read as binary, the signed ext type is exposed as an
unsigned subtype 0..255 (-1 reads as 255), Timestamp is not interpreted,
subtypes are always written as ext/fixext, and subtypes above 255 throw
out_of_range.415. Add the 0xC1 row to the deserialization table, note
that duplicate keys keep the last value, and correct the NaN/Infinity
explanation, the "31 bytes / 15 elements" wording, the ill-formed UTF-8
sentence, and the generic 0xC1-key error message.

Add fixext16 and the unsigned subtype mapping to binary_t.md and
binary_values.md, reword "maximum of the MessagePack ext type" in
to_msgpack.md and exceptions.md, and mark MessagePack deserialization
as incomplete (non-string keys) in the binary formats overview.

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

* Address review of the MessagePack documentation

Name parse_error.113 for a 0xC1 map key, move the duplicate-key sentence out of the ext section, and keep the 0xC1 row in marker order.

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

---------

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

docs/mkdocs/docs/api/basic_json/to_msgpack.md

@@ -53,7 +53,7 @@ Strong guarantee: if an exception is thrown, there are no changes in the JSON va
   value, array, or object exceeds 4294967295, the maximum MessagePack can store; example:
   `"MessagePack length 4294967296 exceeds maximum of 4294967295"`
 - Throws [`out_of_range.415`](../../home/exceptions.md#jsonexceptionout_of_range415) if the subtype of a binary value
-  exceeds 255, the maximum of the MessagePack ext type; example:
+  exceeds 255, the largest value the library can store in the one-byte MessagePack ext type; example:
   `"subtype 70000 is too large for the MessagePack ext type (max 255)"`
 - Throws [type_error.316](../../home/exceptions.md#jsonexceptiontype_error316) if a string or object key in `j` is
   not valid UTF-8 and `error_handler` is `strict`