Reorganize `Hosting Documentation`

From the slack discussion:

Shuhei Kadowaki 4 minutes ago By the way, the section Authentication: SSH Deploy Keys should be in the parent section of GitHub, not in the Travis CI section? It often confuses me that I need to jump to the Travis section when I try to read through the instructions for GitHub actions (and I guess most people use GH actions more than Travis these days?)

Morten Piibeleht 2 minutes ago Yeah, that's an artifact from the Travis days :older_adult: In principle, that page should be reorganized and rewritten.

The main goal of this issue to make it easier to read through the instructions for GitHub Actions. Currently it is a bit confusing since a part of the instructions currently redirects to the Travis section, that is used rarely these days.