Add Doxygen comments with URLs to public API

[nlohmann]

Every public API should be annotated with a URL to the documentation. This can help us to detect undocumented functions in the future.

[nlohmann]

This has since been implemented extensively: include/nlohmann/json.hpp now contains over 200 @sa https://json.nlohmann.me/api/... documentation links across the public API surface, so this suggestion appears to have already been acted on since the discussion was opened.

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This reply was drafted by Claude Code on behalf of @nlohmann, as part of a review of open discussions.