Sphinx doc for gitlab page with CI

Related to issue #28 (closed)

This merge request adds the initial documentation for the tauc repo created with sphinx. To create a gitlab page for the tauc repo, one creates a html from a ReSTructure file (.rst, a kind of markdown) with the software called sphinx. Gitlab can made that automatically with a CI pipeline, since there is a sphinx runner already available. When merging a branch to the master in the tauc repo, the gitlab-ci.yml file triggers the build and make of the html automatically (always, even if the merge does not include any modification of the gitlab pages related file, that is, the .rst). The resulting gitlab page can be seen here: http://mipdata.gitlab-pages.dkrz.de/tutorials-and-use-cases/.

List of changes:

1. create docs/ folder with the sphinx config
2. add the gitlab-ci.yml config CI as said in https://www.dkrz.de//up/systems/gitlab-runner
3. update .gitignore