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>

docs/mkdocs/docs/home/exceptions.md

@@ -1153,7 +1153,7 @@ A JSON Patch `move` operation's `"from"` location is a proper prefix of its `"pa
 ### json.exception.out_of_range.415
 
 MessagePack's ext type and BSON's binary subtype are each stored in a single byte. This exception is thrown when serializing a
-[`byte_container_with_subtype`](../api/byte_container_with_subtype/index.md) whose subtype exceeds 255.
+[`byte_container_with_subtype`](../api/byte_container_with_subtype/index.md) whose subtype exceeds 255, the largest value the library can store in that byte.
 
 !!! failure "Example message"