* update quickstart and remove os import
* simplify theme use
* list sphinx_rtd_theme as extension
Our docs builds failed last night, presumably because #9754 updated `sphinx_rtd_theme` which changed some unknown thing.
Looking into it, our usage of this project was very unconventional. Following the code comment I deleted in this PR to https://docs.readthedocs.io/en/stable/faq.html#i-want-to-use-the-read-the-docs-theme-locally, simple instructions are given to put the following in your `conf.py` file:
```
extensions = [
...
'sphinx_rtd_theme',
]
html_theme = "sphinx_rtd_theme"
```
I did this instead of the more complicated logic we were using and all builds passed locally. I also triggered a build on readthedocs with these changes which also passed.
579b39dce1