Add user agent to HTMLProofer

The brew documentation tests use `HTMLProofer` to check for broken
links in generated documentation HTML and this is causing all brew PRs
to fail CI because one of the linked pages will return a JavaScript
challenge page if the request uses a user agent that the server
doesn't like (or no user agent at all). The default `HTMLProofer` user
agent is "Mozilla/5.0 (compatible; HTML Proofer/5.2.1;
+https://github.com/gjtorikian/html-proofer)" and, for whatever
reason, the Mozilla user agent triggers the upstream server's scraper
safeguards.

This updates docs/Rakefile to use a generic `HTMLProofer/5.2.1` user
agent for these requests, which addresses this particular failure.
I've set this up to conditionally prepend `HOMEBREW_USER_AGENT` if
it's available in the environment, though it's not currently present
in this context.