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/parsing/untrusted_input.md

@@ -50,19 +50,13 @@ stack, however deeply it is nested.
 
 Once a value has been parsed, operations that walk it recursively -- serializing it with
 [`dump`](../../api/basic_json/dump.md), hashing it, copying it, comparing two values with `#!cpp ==`, `#!cpp <`, or (in
-C++20) `#!cpp <=>`, merging with [`update`](../../api/basic_json/update.md), and applying a
-[`merge_patch`](../../api/basic_json/merge_patch.md) -- descend at most 128 levels on the call stack and continue
+C++20) `#!cpp <=>`, merging with [`update`](../../api/basic_json/update.md), applying a
+[`merge_patch`](../../api/basic_json/merge_patch.md), creating a patch with [`diff`](../../api/basic_json/diff.md) or
+[`merge_diff`](../../api/basic_json/merge_diff.md), [`flatten`](../../api/basic_json/flatten.md), and the binary
+writers (`to_cbor`, `to_msgpack`, ...) -- descend at most 128 levels on the call stack and continue
 below that with an explicit stack instead, so none of them can exhaust the stack either, however deeply the value is
 nested. Destroying a value (its destructor) never recurses at all, regardless of nesting depth, for the same reason.
 
-!!! note "Not every operation is bounded yet"
-
-    [`diff`](../../api/basic_json/diff.md), [`flatten`](../../api/basic_json/flatten.md), and the binary writers
-    (`to_cbor`, `to_msgpack`, ...) still recurse once per nesting level; this is called out as work in progress in the
-    [assurance case](../../community/assurance_case.md#secure-design). A value deep enough to matter for these
-    operations would typically first have to survive parsing without hitting a self-imposed depth limit, as described
-    above.
-
 ## Input size
 
 The library does not limit the overall size of a JSON text; a value nested or wide enough will use memory