Adds CSS styling for doc pages generated by sphinx-click.

Summary:
The doc pages for the Dagster CLI are generated by sphinx-click, which didn't have nice CSS yet.

Before

After

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