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• Removes some weird/old/out-of-date information from the general user guide, which I'll rewrite later.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