docs: add IDs to @requirement tags to fix Doxygen autolinking

[VasuBhakt]

This PR fixes a documentation bug where the word "The" was being incorrectly identified as a requirement ID by Doxygen. This caused Doxygen's autolinking feature to turn every instance of the word "The" in the generated documentation into a hyperlink.

Changes:

• Added unique requirement IDs (REQ-JSON-01 and REQ-JSON-02) to the @requirement tags in iter_impl.hpp and json_reverse_iterator.hpp.
• Updated the amalgamated single_include/nlohmann/json.hpp file.

Why:
Doxygen's @requirement command (often an alias for \xrefitem) expects the first argument to be a unique identifier. Without it, the first word of the description ("The") was used as the ID, leading to global autolinking of that word.

• The changes are described in detail, both the what and why.
• If applicable, an existing issue is referenced.
• The Code coverage remained at 100%. (Documentation only change, no code logic affected).
• If applicable, the documentation is updated.
• The source code is amalgamated.

[nlohmann]

Looks good to me.

[nlohmann]

Thank you!

[VasuBhakt]

Thanks a lot Niels! This was my first PR! Thank you very much!