docs: fix BON8 documentation gaps found in the format review (#5932)

* docs: fix BON8 documentation gaps found in the format review

Verified each claim with small C++ probes before writing it.

- index.md: BON8 serialization is also incomplete for invalid UTF-8.
- bon8.md: document duplicate keys (last wins), the error ids for
  malformed input incl. 0xF5..0xF7 + 0x80..0xBF, exact string
  termination wording, wide integers and discarded values in the
  unsupported list, and title two untitled admonitions.
- exceptions.md: to_bon8 in type_error.316, BON8 in parse_error.112
  (with example) and .113, wide integers in out_of_range.407.
- binary_values.md: bytes 0..39 take one byte in BON8.
- from_bon8.md: note on strict UTF-8 and duplicate keys.

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

* Address review of the BON8 documentation

Restore the correct statement that marker bytes cannot begin a UTF-8 character (0x80..0xBF occur in valid UTF-8 as continuation bytes), and reword two sentences.

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

* docs: drop the discarded-value remark from the BON8 page

Discarded values are not mentioned on the individual format pages.

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

* docs: do not list invalid UTF-8 strings as a BON8 serialization gap

JSON text must be valid UTF-8, and to_bon8 rejects ill-formed strings by design (type_error.316), as documented on the BON8 page. Restore the original cell and realign the table.

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

---------

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

docs/mkdocs/docs/api/basic_json/from_bon8.md

@@ -74,6 +74,11 @@ Strong guarantee: if an exception is thrown, there are no changes in the JSON va
 - Throws [parse_error.112](../../home/exceptions.md#jsonexceptionparse_error112) if a parse error occurs, for instance
   an invalid byte, a string that is not valid UTF-8, or an object key that is not a string
 
+The UTF-8 validity of strings is always checked, independent of
+[`JSON_STRICT_BINARY_UTF8`](../macros/json_strict_binary_utf8.md), because the end of a BON8 string is determined from
+its encoding. Duplicate object keys are accepted; the last value wins (see
+[BON8](../../features/binary_formats/bon8.md)).
+
 ## Complexity
 
 Linear in the size of the input.