documentation/robots.txt: Disallow Past Releases Documentation

It was noticed that Google/DDG sometimes are sending users to the past
releases documentation. To avoid misinformation, disallow this archive
from the search engines.

This robots.txt only covers docs.FreeBSD.org and not www.FreeBSD.org, so
also remove unnecessary items.

Differential Revision: https://reviews.freebsd.org/D31459