Provide ways for users to advertise willingness for us to test their website

[cscheid]

We want to ensure quarto updates don't break existing projects. One way to do this would be to let users advertise in their _quarto.yml files that they're opting in to us compiling their projects.

For example, we could add a opt-in-to-quarto-dev-testing: true option. This is relatively easy to search for in GitHub code search, and would signal to quarto devs that the quarto project owners actively want this kind of testing.

This is only feasible if the project uses GHA workflows to render. This way we can know for sure that the project has all necessary assets, etc. Nevertheless, it would be a good way for authors to communicate their willingness to add their project to our test suite, so to speak.