[docs] Put DeveloperPolicy under "Development Process Documentation"
[docs] The GEP FAQ is not "design and overview"
[docs] Pull IRC and Mailing Lists under a new "Community" heading.
[docs] Reorganize mailing lists.

Order them roughly by "which one should a newbie join first".
[docs] Annotate mailing lists with their "name".

Nobody says "the developer's list" or "commits archive"; they always say
"llvmdev" or "llvm-commits". It makes sense for our documentation to
at least make that association explicitly.