Add steps to export CSS to help page

The steps outlined in [this issue](https://github.com/darkreader/darkreader/issues/2538#issuecomment-663756505) seem to answer a frequently asked question in the issues and would make a great addition to the help page.

Start with the steps described in the linked issue comment and locate the help page in the darkreader.org repository. Add the CSS export instructions there, preserving the page’s existing style; done means the frequently asked question is answered with those steps.

Written by the indexing model from the issue text.