Add related projects

[m-aciek]

Since we now have the translations.python.org domain, we should link to the PyPI translation project at least. Optionally also to other translation projects from the Python ecosystem, like scientific projects, Django and microbit.

Not sure if it should be added as a new item (and page) in navbar or in the footer.

[m-aciek]

Hm, or even we could include the PyPI translation in the completion as well?

[m-aciek]

Or add a separate completion bar for PyPI. (Sorry, compulsive brainstorming.)

[hugovk]

Links to other projects is fine, but I think it'd be better to keep this dashboard focused on completion of python.org projects.

[StanFromIreland]

Yes, I'm leaning towards a section in the devguide in this case.

[m-aciek]

packaging.python.org could have a separate completion bar? Also we currently don't account for the Sphinx theme translation in the overall progress, but that's probably of a marginal impact.