Read The Docs should not use relative paths for assets

[vbrown608]

Read The Docs builds the theme for the documentation using relative paths, ex <link rel="stylesheet" href="_static/css/theme.css" type="text/css" />. This causes these assets to 404 when the browser loads certbot.eff.org/docs instead of certbot.eff.org/docs/. We should see if we can configure Read The Docs to output the correct absolute path.