Enable multi-version documentation

[jorgepiloto]

🐞 Problem
Once the CI/CI takes advantage of pyansys/actions, see #1632, it will be possible to use the multi-version documentation. The following tasks need to be addressed:

• Create a gh-pages branch within this repository and enable it in the settings.
• Create a release/ folder containing a versions.json file.
• Update to the latest 0.7 ansys-sphinx-theme and configure the conf.py file.
• Verify HTML, PDF and JSON builders.
• Cherry pick previous changes into the latest two stable releases to clutter the multi-version drop-down.
• Remove development CNAME and doc repositories.

[jorgepiloto]

First, #1632 and #1104 must be resolved.

[jorgepiloto]

Pinging here @germa89, @RobPasMue and @clatapie. Hopefully, we can implement this next week.

[RobPasMue]

Most def!