Adds CSS styling for doc pages generated by sphinx-click.
The doc pages for the Dagster CLI are generated by sphinx-click, which didn't have nice CSS yet.
Unfortunately, some the doc text in "After" would look nicer if it was monospaced. For example, in the docs for dagster pipeline backfill:
dagster pipeline backfill -f /path/to/file.py -a define_some_repo -p <<pipeline_name>>
It's a bit tricky since the click package outputs the help text into the terminal when you run dagster --help.
Test Plan: Render documentation site locally && buildkite
Reviewers: sashank, yuhan
Reviewed By: sashank
Differential Revision: https://dagster.phacility.com/D4872