Consider using a sub domain for the docs

[codeliner]

Follow up of #63

The idea is to use http://docs.getprooph.org and point it to the gh-pages branch folder docs/html
We need to check if that is possible with github pages.

[codeliner]

I've created a new repo: https://github.com/prooph/docs to host the complied docs
The sub domain docs.getprooph.org is configured.

@prolic You need to add a CNAME record for docs.getprooph.org that points to prooph.github.io.
See: https://help.github.com/articles/setting-up-a-custom-subdomain/

As soon as the new sub domain is working I'll compile a redirect into the docs and push it to the proophessor repo so that old links are redirected to the new sub domain.

[prolic]

@codeliner done.