A more recent request for this feature has been made via the devel mailing list:

https://lists.gnupg.org/pipermail/gnupg-devel/2018-March/033547.html

Suggestions include adding <link/> elements to relevant pages:

https://lists.gnupg.org/pipermail/gnupg-devel/2018-March/033550.html

This can be done with org-feed.el, but I am unsure whether the cut down build system for gnupg.org includes org-feed.

This does not require org-feed.el as far as I can tell, but it does require components of current Org Mode HTML export and publishing features which do not appear to be available in the current gnupg.org website build system.

In a current version of Org Mode (I'm currently running version 9.1.7 with the contrib-plus packages) the [X]HTML export will use this header:

#+HTML_HEAD_EXTRA: <link type="application/rss+xml" href="https://git.gnupg.org/cgi-bin/gitweb.cgi?p=gnupg-doc.git;a=rss;f=web/index.org"/>

To produce this additional HTML between the <head/> tags:

<link type="application/rss+xml" href="https://git.gnupg.org/cgi-bin/gitweb.cgi?p=gnupg-doc.git;a=rss;f=web/index.org"/>

Testing exactly this in the index.org page on preview.gnupg.org did nothing.