Create a page for Volto CI

[stevepiercy]

And now that I think about it, most of the new-to-GitHub users have no idea of what is CI and how to use it. I think we should have either a new section or page specifically for CI, just as we have separate pages for each thing that a contributor needs to know. I think this is worth a separate issue and PR itself, so that we don't wander too far off course in this PR.

I think it should be a new subsection under Developing Volto.

Originally posted by @stevepiercy in #5882 (comment)