Redirect for `textbook.textpattern.net`

[petecooper]

From an email communication:

Textpattern forum has some old links that look like this:
http://textbook.textpattern.net/wiki/index.php?title=Txp:if_article_category
Suggestion: create the subdomain textbook.textpattern.net
and redirect from there every request to https://docs.textpattern.com/