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/basic_json/operator_lt.md

@@ -15,7 +15,8 @@ bool operator<(ScalarType lhs, const const_reference rhs) noexcept(/* see below
   following rules:
     - If either operand is discarded, the comparison yields `#!cpp false`.
     - If both operands have the same type, the values are compared using their respective `operator<`.
-    - Integer and floating-point numbers are automatically converted before comparison.
+    - Integer and floating-point numbers are compared by their exact values, without converting the integer to
+      floating-point. For instance, `9007199254740992.0` is less than `9007199254740993`.
     - In case `lhs` and `rhs` have different types, the values are ignored and the order of the types
       is considered, which is:
         1. null