Update Phame documentation to reflect changes to the application

Summary:
Ref T9360. Old docs felt a little weird to me (particularly very-old text like "favoring the individual rather than the collective").

Try a simpler tone focused more on use cases and examples?

Test Plan:
Read documentation.

Also, viewed a post list and saw monograms.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9895, T9360

Differential Revision: https://secure.phabricator.com/D16246