Add a symlink for each branch in online docs

[François-Xavier Coudert]

The documentation for different releases of GCC in the same branch
vary only slightly. For that reason, I think it would be nice to be
able to have a URL for "the doc of a given branch", meaning "the doc
of the last released version of this branch". For example, that would
be achieved by creating a symlink
http://gcc.gnu.org/onlinedocs/gcc-4.2 ->
http://gcc.gnu.org/onlinedocs/gcc-4.2.1

Would that be considered useful? And if yes, how to implement it?

FX