Generate redirects for aliases

[lionel-]

When ?foo is an alias of ?bar, the R help redirects foo to bar automatically. This means aliases and topics are interchangeable and changing which is what is backwards compatible. However, this is not the case on pkgdown sites because aliases do not get an html file.

It seems that pkgdown links in the larger community will degrade over time as package authors rename their topics and add compatibility aliases. Since the internal help system of R redirects aliases automatically to the right topic, users would expect the same for their pkgdown site and won't necessarily think to check that the links still work. Also since the links are often created in other pkgdown sites through cross-linking, there's even less reason for them to check. It would be useful to get automatic generation of redirects for aliases to prevent URL degradation.