use redirect pages without version number that point to latest release

[mitzimorris]

Right now with every release we need to update version numbers for documentation - cf., this commit:

https://github.com/stan-dev/stan-dev.github.io/commit/6ff23538589a53e7a429d3b519d07286f3a065fd

instead, index pages should link unversioned page, which redirects to latest version. updating this link can be scripted as part of the release process or similar.