Throw type_error.321 for discarded values in to_bon8 (#5802)

write_bon8_value silently skipped discarded values, but the array/object
count marker still counted them, so [1, discarded] produced 82 91: a marker
announcing two elements followed by one. Throw type_error.321 like the other
binary writers do, at any nesting level.

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

docs/mkdocs/docs/home/exceptions.md

@@ -817,7 +817,7 @@ does not list an enumerator and it is therefore converted like the first listed
 A discarded value (one created by [`parse()`](../api/basic_json/parse.md) with a callback that returns `false` for the
 value, or by default-constructing a [`basic_json`](../api/basic_json/index.md) with
 [`value_t::discarded`](../api/basic_json/value_t.md)) was passed to a binary serialization function, either directly or
-nested in an array or object. There is no way to represent a discarded value in CBOR, MessagePack, UBJSON, BJData, or BSON.
+nested in an array or object. There is no way to represent a discarded value in CBOR, MessagePack, UBJSON, BJData, BSON, or BON8.
 
 !!! failure "Example message"