faq: Add more sections, rework older ones

Remove or updated outdated information, fix grammar, and import most of the FAQ from the wiki page: https://wiki.postmarketos.org/index.php?title=FAQ&oldid=42466

After this is merged, we should remove all the content from the FAQ wiki page and explain that the FAQ now resides on postmarketos.org, and change the FAQ link on the wiki homepage. Maybe also block editing of the FAQ article from non-admin users?