Hyperlinks for source file references

Commit b73c3a11963 introduced <ulink url=xyz><filename>xyz</filename><ulink>
hyperlinks for files in the postgres source tree by linking to the gitweb
interface at git.postgresql.org. This, IMO, makes the content referred to more
accessible, and is especially interesting for README files. The links in
question can be found here:

https://www.postgresql.org/docs/devel/tableam.html

Right now we have ~85 references to source files using <filename /> in the
docs, should we convert them to hyperlinks like the ones in the Table AM docs?
I would be happy to make a patch that implements this, but I wanted to check
here before making the effort to make sure it's not immediately nayed.

This does of course present a question for the backbranches, pointing to the
HEAD version in the docs for a previous major release isn't entirely accurate.
On the other hand, we already do that today in the above tableam page so if we
don't like that we might as well sort out now what to do about that.

--
Daniel Gustafsson