docs: now has icon and no redundant `- Sourcegraph` in <title> on home page

Created by: ryan-blunden

The previous home page title was Sourcegraph documentation - Sourcegraph docs which looked weird in Google. Now it is just Sourcegraph documentation.

The error page now just has the page title Error, not Error - Sourcegraph docs which is fine because it's not indexed by search engines.

Browser tab now has Sourcegraph logo