Merge and cleanup documentation

There is docs/ and backend/docs and backend/docs-cp

As we no longer have frontend and backend separated, it makes IMHO no sense to have different documentation places.

Please remove old documentation which is no longer valid and merge them.

[certrik]

@davewood Would be great if you could do it. If you find the time.

[MarkHofstetter]

for further complication: there is also
https://github.com/certat/do-portal/blob/master/docs/01_INSTALL.md

which already includes the docker documentation