Use travis deploy instead of doctr to deploy documentation

Travis documentation jobs started to fail recently. This was first observed in https://github.com/sardana-org/sardana/pull/1214#issuecomment-540885289 and from then on affects all the other builds. Apparently nothing had changed: Docker image, doctr had not changed around this dates. The investigation on this issue is quite difficult. The job builds either report: git stash --all failed: 1 or simply hangs.

As an alternative solution I have tried to directly use travis deploy to GitHub Pages - therewith this PR.

There are two important issues to be considered now:

1. Other, different versions of the documentation won't be automatically build. But IMHO this is not really a problem, actually I consider it as an improvement. We have never used them in the last 1 year and a half and now we need to clean them up - see this page. So, basically instead of manually removing documentation versions now will need to manually add them. To add a new version, a new project will need to be added in sardana-controls organization and travis.yml would need to be updated with the new deploy.
2. Now doctr uses SSH key to push to sardana-doc project. The travis deploy needs a personal token. If we add a personal token, for examply my personal token, it will have rights to push to any of the repository that I have access. To overcome this issue a bot user could be created with right to push to sardana-doc repository only (and eventually other documentation versions).

Still pending to be done:

• solve point 2 mentioned above
• uncomment other jobs (now commented out to speedup tests)

Meanwhile I have used my personal token and push it to my fork of sardana-doc. One can check the results on https://reszelaz.github.io/sardana-doc/