Port ph2acf-docu-dev to central repo
This is the promised port of the inner tracker user guide currently hosted at https://ph2acf-docu-dev.docs.cern.ch/ to the central documentation pages. I've largely just copied over the files from @bristic and @tharte, applying some smaller fixes.
Since this is a large addition to the documentation, my proposal would be to merge this and then apply fixes. I've pushed directly to the it_calib_docs branch in case @dinardo would like to apply corrections right away. There are probably several outdated statements here. Still, better to have this than nothing or only the slides. We should see this as community-developed documentation.