Make vector recycling rules its own FAQ

[DavisVaughan]

Closes #1877

Tricky due to r-lib/pkgdown#1876

I was going to update our vector_recycling_rules page in vctrs to a true faq page with @name of theory-faq-recycling to match the other faq pages and use @aliases vector_recycling_rules to retain the old link. But since vector_recycling_rules won't get an html page since it is just an alias, that means the places in dplyr where we link to that page will be broken until dplyr rebuilds the site with the new version of vctrs, and I don't want that.

So instead I've kept vector_recycling_rules as the main @name.

To work around the pkgdown issue, I used a manual redirect in _pkgdown.yml to ensure vector_recycling_rules.html always exists

[olivroy]

Maybe the pkgdown redirects can help here? https://pkgdown.r-lib.org/news/index.html?q=redirects#new-features-2-0-0

[DavisVaughan]

@olivroy neat!