Collect statuses of downstream devtesting

[bsipocz]

This issue separates out the testing parts of #304

One approach is to advocate for downstream users to include testing against the dev version of pyvo in their CI.
If they add that test into a separate workflow, then it's easy to obtain a status badge for the dev tests. We could then collect these statuses onto a dashboard.

Here is how I did it for astroquery: https://github.com/astropy/astroquery/blob/main/.github/workflows/ci_devtests.yml

And the badge it creates:

The pre-release testing is also added to the tox file (see https://github.com/astropy/astroquery/blob/main/tox.ini), it can be used as a template for others (though I think if a package/notebooks do thorough testing for the pyvo dev version, they don't really need to test any possible pre-releases as here there aren't really diversion between releases and the main branch)

[bsipocz]

There are a few pointers about badges, and that they can be generated for separate workflows (but not for separate jobs in a workflow) here: astropy/astroquery#2378

[bsipocz]

And here is the CI for a notebooks repository https://github.com/NASA-NAVO/navo-workshop, with the badge for the dev version testing:

[bsipocz]

So, here the action item is to work these examples into a docs page, and suggesting others to use them as quasi templates to set up a basic test matrix, and a job that in fact uses pyvo's dev version.

[bsipocz]

We agreed on not to collect these in the pyvo docs, and I think the resources linked in this issue is enough to showcase how to generate the badges, so I go ahead and close the issue.