It doesn't really make sense to publish multiple versions of
contributor documentation, but when contributor documentation is
included in branched projects this is precisely what we end up with.
Luckily, the recent work to standardize project contributor docs
puts them under a consistent subtree of the each project's
documentation, so we can easily match on them.

In order to avoid things like web search results and stale bookmarks
sending unwary users to outdated versions of contributor
information, just always redirect any old series to "latest"
instead.

Change-Id: If55ec3d4e0c98e43e985e293f23813794315450d