Add docs about sphinx-autobuild

[jdillard]

Now that sphinx-autobuild is part of the sphinx-doc organization, I thought it would be useful to highlight it's usage in the docs.

Previews:

• https://sphinx--13011.org.readthedocs.build/en/13011/usage/quickstart.html#running-the-build
• https://sphinx--13011.org.readthedocs.build/en/13011/internals/contributing.html#contribute-documentation

Feature or Bugfix

• Feature

Purpose

It was mentioned in the contributing section, but I thought it would be more useful to also mention in the getting started section as previewing changes is a common question for people new to docs-as-code.