Separate new docs

[gaearon]

This moves a bunch of old docs into docs/docs-old as a temporary measure so we can focus on the new ones. The new ones are in docs/docs. The sidebar looks like this now:

Below it, there’s still all old content so we can quickly look at it when necessary:

My intention is that we remove the old guides and "tips" one by one but make sure that every time we delete a file, we have all its content somewhere in new files. We can mostly move Reference files as is into the new docs but it would be nice to fix staleness when we see it.

At the end, we should end up with just one docs folder, and the old content gone. Then we’ll do a separate pass to set up redirects.

Sounds reasonable?