Add Sphinx setup and virtualenv for documentation build support

[shinde-rahul]

This adds a DDEV configuration that has

• sphinx and required dependencies.
• a post-start hook that builds the documentation

After editing, you can ddev build-docs to update the html and view in a browser.

[rfay]

I didn't manually test, but this strategy seems fine.

[adiati98]

I've tested this locally and it worked!!! 🚀

Thanks so much, @shinde-rahul! ✨