docs: add IDs to @requirement tags to fix Doxygen autolinking (#5162)

Signed-off-by: Swastik Bose <cpswastik31@gmail.com>

Author: VasuBhakt

include/nlohmann/detail/iterators/iter_impl.hpp

@@ -35,7 +35,7 @@ This class implements a both iterators (iterator and const_iterator) for the
       been set (e.g., by a constructor or a copy assignment). If the iterator is
       default-constructed, it is *uninitialized* and most methods are undefined.
       **The library uses assertions to detect calls on uninitialized iterators.**
-@requirement The class satisfies the following concept requirements:
+@requirement REQ-JSON-01 The class satisfies the following concept requirements:
 -
 [BidirectionalIterator](https://en.cppreference.com/w/cpp/named_req/BidirectionalIterator):
   The iterator that can be moved can be moved in both directions (i.e.

include/nlohmann/detail/iterators/json_reverse_iterator.hpp

@@ -29,7 +29,7 @@ namespace detail
 iterator (to create @ref reverse_iterator) and @ref const_iterator (to
 create @ref const_reverse_iterator).
 
-@requirement The class satisfies the following concept requirements:
+@requirement REQ-JSON-02 The class satisfies the following concept requirements:
 -
 [BidirectionalIterator](https://en.cppreference.com/w/cpp/named_req/BidirectionalIterator):
   The iterator that can be moved can be moved in both directions (i.e.

single_include/nlohmann/json.hpp

@@ -13807,7 +13807,7 @@ This class implements a both iterators (iterator and const_iterator) for the
       been set (e.g., by a constructor or a copy assignment). If the iterator is
       default-constructed, it is *uninitialized* and most methods are undefined.
       **The library uses assertions to detect calls on uninitialized iterators.**
-@requirement The class satisfies the following concept requirements:
+@requirement REQ-JSON-01 The class satisfies the following concept requirements:
 -
 [BidirectionalIterator](https://en.cppreference.com/w/cpp/named_req/BidirectionalIterator):
   The iterator that can be moved can be moved in both directions (i.e.
@@ -14566,7 +14566,7 @@ namespace detail
 iterator (to create @ref reverse_iterator) and @ref const_iterator (to
 create @ref const_reverse_iterator).
 
-@requirement The class satisfies the following concept requirements:
+@requirement REQ-JSON-02 The class satisfies the following concept requirements:
 -
 [BidirectionalIterator](https://en.cppreference.com/w/cpp/named_req/BidirectionalIterator):
   The iterator that can be moved can be moved in both directions (i.e.