Update, reorganize and clean up the openidc guide. Use Google as a
concrete IdP example. Use the systemctl command to modernize the service
management commands. Add examples of configuring all required endpoints
in Apache to mirror the new section on configuring protected endpoints
in the main guide and replace the lost examples from the consolidated
WebSSO guide. Remove use of ``a2enmod`` since the Mellon module is
automatically enabled by the package on all supported distros.
Closes-bug: #1793374
Change-Id: Ie5dc4899beff77f121cc62bc8d56763c7671ecc3
ec7f8b95b3