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/home/exceptions.md

@@ -889,7 +889,9 @@ contains other characters (e.g., `1a`), or `-` where it cannot be used.
 
 ### json.exception.out_of_range.405
 
-The JSON Patch operations 'remove' and 'add' cannot be applied to the root element of the JSON value.
+The JSON Patch operations 'remove' and 'move' cannot be applied to the root element of the JSON value. The exception is
+also thrown when the parent or a reference token of an empty JSON Pointer is requested, for instance by
+[`back`](../api/json_pointer/back.md) or [`pop_back`](../api/json_pointer/pop_back.md).
 
 !!! failure "Example message"
 
@@ -914,15 +916,23 @@ double-precision number when `number_float_t` is `#!cpp float`.
 
 ### json.exception.out_of_range.407
 
-This exception previously indicated that the UBJSON and BSON binary formats did not support integer numbers greater than
-9223372036854775807 due to limitations in the implemented mapping. However, these limitations have since been resolved,
-and this exception no longer occurs.
+An unsigned integer number greater than 9223372036854775807 cannot be represented in the binary format. This exception
+is only thrown by [`to_bon8`](../api/basic_json/to_bon8.md), as [BON8](../features/binary_formats/bon8.md) has no
+unsigned 64-bit integer type.
 
-!!! success "Exception cannot occur any more"
+!!! note "Former occurrences"
+
+    The UBJSON and BSON writers used to throw this exception as well:
 
     - Since version 3.9.0, integer numbers beyond int64 are serialized as high-precision UBJSON numbers.
     - Since version 3.12.0, integer numbers beyond int64 are serialized as uint64 BSON numbers.
 
+!!! failure "Example message"
+
+    ```
+    integer number 18446744073709551615 cannot be represented by BON8 as it does not fit int64
+    ```
+
 ### json.exception.out_of_range.408
 
 The size of an array or object in a [binary format](../features/binary_formats/index.md) exceeds the maximal capacity: