Add a new section that lists implemented specs for middleware

Currently, we cannot differentiate between implemented and approved
specs for keystoneclient or keystonemiddleware. Created a directory
to move the completed specs to, and created a new section on the
landing page.

Change-Id: I8d36af36586cd6e42bd58a6b1ff5a9337d5728b8
Merge "Add a new section that lists implemented specs for middleware"