Add custom styles to HTML documentation

Following !296 (merged), 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