Rolling docs build at external server

[c-hofer]

So far we have to build docs locally and push them to gh-pages.

Wanted new behavior:

1. docs are updated automatically with each push of master

2. there remains a possobility to build docs locally in some gitignored folder for testing befor pushing

@rkwitt Research if u want to. i'll do the coding.

[rkwitt]

Totally agree - This would be the desired behavior.