Improve Publishing to GitHub doc section

Happy New Year!

This PR suggests some improvements to the Publishing on GitHub section of the doc.

This work comes from me wanting to publish my Pelican site as User Pages, and not finding the "User Pages" section very helpful.

I think this PR fixes that, and brings other improvements:

* add links to GitHub documentation pages,
* provide more detailed explanations,
* mention the `github` Makefile target,
* explain how to use `ghp-import` for Project Pages as well as for User Pages,
* give information on setting a custom domain.