[PATCH] Add CSS to support discoverable ids in the public documentation website

Dear PostgreSQL website maintainers,

commit e2922702a3 added the XSLT transformation to add links (<a
href="#...">#</a>) that make elements with ids discoverable to the
postgresql docs. I expect the links to (unconditionally due to the lack
of CSS support for now) appear in the devel-docs with the next nightly
build.

As suggested in [1] I've created a patch for pgweb that adds the CSS to
hide the links and display them when hovering the element they are
referring to.

Since I wasn't fully aware of the correct procedure, I initially sent
the patch to pgsql-hackers but I've learnt that it should be sent to
this list instead.

Attached is the (unmodified) patch I initially sent to pgsql-hackers in
[2]. Please see the thread in pgsql-hackers for the discussion so far.

Regards,
Brar

[1]
https://www.postgresql.org/message-id/20230323093555.3icw7zetwtmtrrzu@alvherre.pgsql
[2]
https://www.postgresql.org/message-id/91edf03b-7b63-67b6-7f44-bfeb48e97056@gmx.de