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/features/binary_values.md

@@ -216,7 +216,7 @@ array of integers.
     0x87                                     // object with 1 member
         0x62 0x69 0x6E 0x61 0x72 0x79        // "binary"
         0x84                                 // array with 4 elements
-            0xC3 0x22 0xC3 0x56 0xC3 0x12 0xC3 0x16  // content (each byte as a 2-byte integer)
+            0xC3 0x22 0xC3 0x56 0xC3 0x12 0xC3 0x16  // content (each byte, here all above 39, as a 2-byte integer)
     ```
 
     Note that the subtype is lost, and deserializing `v` would yield the following value: