Improve manual

[jackkamm]

#294 converted the Readme to an Info manual, and did some re-organization, but there are still some improvements to be made both in content and formatting. For example:

1. Add cindex and vindex decorations so that Info-index is useful when viewing the Info manual in Emacs.
2. Update old information (e.g. "Compatible caldav servers"?)
3. Reduce verbosity and make the style/tone more "manual-like". E.g. the "little CalDav rant" section, while interesting, should probably be removed (we could archive it to doc/org-caldav.org_archive, extracting just the relevant details for the manual).

I suggest to do this incrementally (e.g. by section) so that the task is not too large and we can make gradual progress. PRs welcome.