Docs: remove outdated and complex code and dependencies

[humitos]

This commit reduces the complexity of the documentation environment by removing the requirement of having Django installed and loaded to build the docs.

• Remove the djangodocs and doc_extensions: they load the whole Django application unnecesarily. We need to have a bunch of requirements installed just because of this.

• Remove defaults from "Interesting settings" because all the default were wrong. The Django extension was loading an invalid set of settings. This page could be completely removed as well probably.

• Remove featureflag role because of the same reason (loads a whole Django project). We were using it only to show a small description of the feature flags. I wrote that text directly in the page. Feature flags are deprecated and this page will probably deleted soon as well.

• Remove buildpyversions role also. Same reason. Manually wrote the versions supported on each Docker image. This is used for the legacy images, tho. They won't change since we don't maintain them anymore.

Since we don't depend on Django anymore we can remove the -r pip.txt dependency from the documentation. This makes the documentation just to rely on Sphinx and some other extensions only simplifying the environment a lot and running fast.

My thinking behind all of this was that it is not worth to maintain such a complex environment and tools just to be able to output the "default values" from some Django settings in the documentation. In particular, if these defaults are not really useful for our users. Even more, if the default values we are showing to them are wrong and don't reflect the real ones when using the Docker development environment 🙃

NOTE: this branch is based on #9978 because I found this problem while working on that setting simplification. Merging this PR will make #9978 to pass the tests.

---

📚 Documentation previews 📚

• User's documentation (docs): https://docs--9981.org.readthedocs.build/en/9981/

• Developer's documentation (dev): https://dev--9981.org.readthedocs.build/en/9981/

[ericholscher]

This makes a bunch of sense to me. I've never been a huge fan of these dynamic features given how little we use them.

If we want dynamic stuff like this, it would be much better to build this functionality into our platform, so we can do more dynamic things via an API, and update that data eg; on deploy or merge to main.

But, I think we'd all agree that is likely not super important unless we have a lot of users requesting it, and can just move forward with static information in the docs 👍

[ericholscher]

😍

[benjaoming]

THANK YOU 🙏

This will be a big benefit for current documentation efforts!