ENH - Update translations workflows

[trallard]

This PR supersedes https://github.com/pydata/pydata-sphinx-theme/pull/1294 and is the last missing piece to get rid of nox and the `noxfile.

Summary:

• Adds all the needed babel commands to our tox.ini file
• Updates both user and contributor documentation
• Removes unnecessary bits from the CI workflows

Questions:

• https://github.com/pydata/pydata-sphinx-theme/pull/1294 renames the translation files from sphinx.pot to messages.pot which I agree with and aligns best with best practices but I did not change it here since I do not know what the implications are for Transifex @12rambau you might be able to give more insights.
• Something I still do not understand fully is why we do the compilation of the mo files at the same time with compile our other JS assets (see https://github.com/trallard/pydata-sphinx-theme/blob/e9b8d72b6d2d9412f878d3ffd10915a0d23f9e65/webpack.config.js#L22) so perhaps this should be decoupled? And we'd just need to ensure that the files are accordingly updated (maybe through CI) - Edit: I have now removed the PO compilation from webpack.config