Docs feature: auto-build documentation

### What do you see as an issue? It currently takes a few minutes each time to build the docs. So, each small mistake requires a few minutes to wait for the docs to re-build. ### Solving the problem I tried using [sphinx-autobuild](https://github.com/executablebooks/sphinx-autobuild), but I haven't been able to get it to work. ### Anything else I'd be happy to work on a PR with someone who knows restructured text/sphinx better than I do. ### Are you willing to submit PR? - [ ] Yes I am willing to submit a PR! ### Code of Conduct - [X] I agree to follow this project's [Code of Conduct](https://github.com/apache/airflow/blob/main/CODE_OF_CONDUCT.md)