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/api/json_pointer/parent_pointer.md

@@ -12,7 +12,8 @@ Parent of this JSON pointer; in case this JSON pointer is the root, the root its
 
 ## Exception safety
 
-No-throw guarantee: this function never throws exceptions.
+Strong exception safety: if an exception occurs, the original value stays intact. The function is not `#!cpp noexcept`
+and allocates a new JSON pointer, so it can throw `#!cpp std::bad_alloc`; it throws no library exceptions.
 
 ## Complexity