Fix contradictions between the documentation, the README, and the library (#5825)

- Discarded values: the legacy macro only changes <= and >=; != is true by default
- Out-of-range numbers are rejected while parsing, not stored as NaN
- merge_diff round trip: point to the ordered_json caveat
- ensure_ascii example shows the escaped output
- out_of_range.405 is thrown for "remove"/"move" on the root, not "add"
- CBOR stores binary subtypes as tags
- Integer/float comparison uses exact values, not conversion
- diff, flatten, and the binary writers are depth-bounded
- FAQ: max_digits10 for serialization, trailing commas are supported
- out_of_range.407 is still thrown by to_bon8
- FetchContent_MakeAvailable needs CMake 3.14
- json_fwd.hpp contents, operator< link, RFC 7396 for merge patch
- README: dump() wording, merge_diff, MSYS2/build2, patch comment typos,
  operator[] link, embedded CMake example aligned with the docs

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

docs/mkdocs/docs/api/macros/json_use_legacy_discarded_value_comparison.md

@@ -18,7 +18,8 @@ When enabled, comparisons involving at least one discarded JSON value yield resu
 | `>=`         | `#!cpp true`  |
 | `>`          | `#!cpp false` |
 
-Otherwise, comparisons involving at least one discarded JSON value always yield `#!cpp false`.
+Otherwise, comparisons involving at least one discarded JSON value yield `#!cpp false`, except for `!=`, which is the
+negation of `==` and yields `#!cpp true`.
 
 ## Default definition