merc1er authored May 14, 2020 and freetrader committed May 14, 2020

Following !296, we want the HTML generate docs to match the theme of the website (https://bitcoincashnode.org).

These changes add CSS styling to the HTML pages including color update, custom logo and remove "made with MKDocs" watermark.

Test plan:

* in top level source folder, run `mkdocs serve &`
* view the docs with your web browser of choice at `http://127.0.0.1:8000`