Links to files in the GitHub repo's master branch should be to the files
within the branch being generated. For example, in the v2.1
documentation, links should be to the v2.1 branch contents. (Previously
links were being made to the master branch in all our archived content.)
This creates a problem when we want to remove an obsolete file in the
master branch but can't because older documentaiton incorrectly depends
on it.
This new extension defines a :acrn_file: and :acrn_raw: role that will
create links to the given file within the current commit branch.
This PR also replaces docs with hard-coded links to files in the master
branch with uses of these new roles to create links to files.
Signed-off-by: David B. Kinder <david.b.kinder@intel.com>