Improve documentation with mkdocs

I'm looking at writing up some pmbootstrap documentation for f2fs support, and was wondering about the viability of writing up docs in mkdocs at https://gitlab.postmarketos.org/postmarketos/pmbootstrap/docs instead of putting them up on the wiki.

While it may make sense in the long term to move much of the wiki, the focus of this issue is on consolidating the docs for pmbootstrap. This provides a reasonable initial deployment and is still small enough to revert to the wiki if it turns out to be a terrible idea.

If there is a reasonable chance of getting something like this merged, I'm willing to write up a MR.

Pros

• Static pages are easier to scale and lower maintenance than mediawiki
• Docs can be versioned with code
• Docs can be part of a feature MR

Cons

• Slightly higher barrier to entry to to make a docs MR over making a wiki edit