Add a docs/ folder and move existing documentation there

[eqrion]

We actually have enough random doc files that it probably makes sense to consolidate it to a folder.

@Gankro is this okay, or will it break external references to the user guide?

[Gankra]

It would break the currently very active link in this tweet: https://twitter.com/Gankro/status/1138588645768622080

About 150 people have clicked on that link, many more have seen it.

I think this is fine/reasonable after a grace period of a few weeks..?

[eqrion]

Yeah that totally makes sense. I'll delay this for a couple weeks.

[emilio]

Should we do this now?

[Gankra]

yep, let's go for it