docs: correct JSON Pointer documentation and add tests (#5937)

Fix inaccurate or missing statements about unflatten (third round-trip
failure, empty token, typo), value (type_error.306 before pointer lookup),
contains (pointer overload), parent_pointer and operator<< (bad_alloc),
out_of_range.410 example, template parameter requirements (resize(n)), and
document the URI fragment form and non-unique member names of RFC 6901.
Add tests for the unflatten and value() cases.

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

docs/mkdocs/docs/home/exceptions.md

@@ -1083,9 +1083,14 @@ a JSON pointer exceeds the range of `size_type` (e.g., on 32-bit platforms).
 
     ```
     enum value out of range
-    array index 18446744073709551616 exceeds size_type
+    array index 18446744073709551615 exceeds size_type
     ```
 
+    The second message is thrown, for instance, by `#!cpp json::parse("[1, 2]").at("/18446744073709551615"_json_pointer)`
+    on a 64-bit platform, where `size_type` is `#!cpp std::size_t` and this index is `SIZE_MAX`. An index that does not
+    fit into 64 bits (such as `18446744073709551616`) is not reported as 410 but as
+    [`out_of_range.404`](#jsonexceptionout_of_range404).
+
 ### json.exception.out_of_range.411
 
 A JSON Patch `add` operation cannot be applied because the target location's parent is neither an object nor an array. Per [RFC 6902](https://datatracker.ietf.org/doc/html/rfc6902), an `add` target must reference a member of an existing object or an element of an existing array; a primitive value (string, number, boolean, etc.) cannot receive a new member or element.