5440: remove all links to /start temporarily to avoid confusing user

5440: remove all links to /start temporarily to avoid confusing user
Put in quickstart sections on docs homepage for both 1) public pipeline 2) pipeline developers
Revert changes made in #5090 (home title back to "Arvados Docs", topnav link to "arvados.org")
Refactor /user/index.html, removing references to /start and directing new users to homepage quickstart section