Fix email notifications for lfdocs-conf releases

Unfortunatelly the name of the repo does not match the
configuration in the code and we need to make sure the
email notification announces the corrected name of the
project which should be lfdocs-conf.
Add a new PROJECT_SLUG variable to use in case the name
of the repo is different from its configuration.

Issue: IT-20404
Signed-off-by: Jessica Wagantall <jwagantall@linuxfoundation.org>
Change-Id: I0dc7b0088851ce8abf8238bc63e8e5bdbfc2ea9b