RFC: Repo housekeeping - rename repo

[petecooper]

As an extension to #218, I propose to rename this repo from textpattern.github.io to docs.

For background, the current name (textpattern.github.io) follows a scheme required to use GitHub Pages. As we no longer use that service, the current name is both misleading and a little crufty. Renaming the repo to something more sane like docs makes more sense from a UX perspective.

Considerations:

• We can remove duplicate (or worse, mismatched) content between https://textpattern.github.io/ and https://docs.textpattern.com as the former would no longer work.
• There are currently two 'active' forks with pending changes in the last 12 months: https://github.com/textpattern/textpattern.github.io/forks?include=active&page=1&period=&sort_by=last_updated - specifically https://github.com/rwetzlmayr/textpattern.github.io & https://github.com/vladas-palubinskas/textpattern.github.io - we can manually assist with either of both of these forks' pull request(s) where needed.

What have I missed from the above?

Any advice & feedback gratefully received. Thank you.

Paging @Bloke @bloatware @philwareham @jools-r