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/features/macros.md

@@ -243,9 +243,9 @@ See [full documentation of `JSON_USE_GLOBAL_UDLS`](../api/macros/json_use_global
 
 ## `JSON_USE_LEGACY_DISCARDED_VALUE_COMPARISON`
 
-When defined to `1`, the library restores the legacy behavior in which a discarded value compared equal to itself. This
-behavior is [deprecated](../integration/migration_guide.md#miscellaneous-functions) and switched off (`0`) by
-default.
+When defined to `1`, the library restores the legacy behavior in which comparing a discarded value with `<=` or `>=`
+yields `#!cpp true` instead of `#!cpp false`. This behavior is
+[deprecated](../integration/migration_guide.md#miscellaneous-functions) and switched off (`0`) by default.
 
 See [full documentation of `JSON_USE_LEGACY_DISCARDED_VALUE_COMPARISON`](../api/macros/json_use_legacy_discarded_value_comparison.md).