Adding sphinx package

This MR adds several features

• a new API sphinx reference guide is added in the 'read the docs' style which is deployed with the docs and will appear under https://umami.docs.cern.ch/sphinx-docs once this MR is merged
• the docs are now also built in the MR and deployed as a separate website for each MR under the following URL https://umami-ci-provider.web.cern.ch/mr-docs/{mr_id}
• these URLs are posted to the MR with the umamibot

for this particular MR you can find the new reference guide under https://umami-ci-provider.web.cern.ch/mr-docs/344/sphinx-docs as well as the temporary docs under https://umami-ci-provider.web.cern.ch/mr-docs/344