Remove link to old forum from the release docs
Link to the static API docs and separate out latest docs

Since this is the official release documentation for a version, we should
link to the static API documentation for this version, rather than the
latest online version as there is the potential for confusion.

I've separated the list of links into two categories:
* those of immediate significance for someone using the release; and
* information on the latest release, and contributing to YUI.
Add links to YUI on GitHub
Remove details on src directory from release docs

The release does not actually include the src directory any more so there
is no need to mention it in the list of directories, or how to rebuild
modules using Shifter from it.