Allow for direct URLs to abbreviated legacy PostgreSQL releases

Allow for direct URLs to abbreviated legacy PostgreSQL releases

For example, while a link to "/docs/release/9.1.0/" would work,
going to "/docs/release/9.1/" would not; in this case, it would
show the release details for "9.0.1". While the probability of
someone modifying the links to see the different release notes
is low, we would still need to take action on what is an
incorrect link.

Instead of returning a 404, this transposes the major/minor
version to point at the canonical version number and render the
expected page. This is set up to work for the 6, 7, 8, 9 series.

Reported by: Nikolay Samokhvalov <samokhvalov@gmail.com>