docs: migrate reana-ui readthedocs to docs.reana.io

[mvidalgarcia]

Move CERN integrations locally to new docs.reana.io or another site (DR docs, etc.).

Identify which parts are needed to remain and update content if needed to make it more generic and avoid CERN specificity.

[mvidalgarcia]

Bumping priority as GitLab integration instructions aren't available anywhere ATM.

[tiborsimko]

Something for the developer wiki.

[mvidalgarcia]

Which page/category in the wiki does this section belong to?

[tiborsimko]

This could perhaps go into "Understanding code base > SSO" and "Understanding code base > GitLab integration". We could start by a few hints e.g. what is SSO and that it comes into REANA via Invenio, and then we continue with the recipe how to develop for SSO locally. Ditto for GitLab, we could briefly describe the webhooks setup, and have the recipe section how to test GitLab locally.

(As an example, see the "Workflow complexity" page, that was having only the "how to run in ipython" main body before, and where I added a brief About section to explain the concept.)