Document Calendar imports
Summary:
Ref T10747.
- Adds import documentation.
- Adds import/export docs to the help menu.
- Removes some weird/old/out-of-date information from the general user guide, which I'll rewrite later.
Test Plan: Read documentation somewhat thoroughly.
Reviewers: chad
Reviewed By: chad
Maniphest Tasks: T10747
Differential Revision: https://secure.phabricator.com/D16766