Link to possible Pixelfed instance list, create layered getting started section.

[niccokunzmann]

If there is a (curated) list of pixelfed instances that one can join, it would be nice to have those on the getting started section.

This way, the difficulties are layered:

1. How to join (see the list)
2. How to have an automatic setup/renting (if that exists, like mastohost)
3. How to self-host on a server (is there something like a vercel/heroku/digital Ocean link?)
4. How to self-host on your own hardware (easy setup with YuNoHost)
5. How to do the full setup (documented + docker #142 )