Add simple CLI page to documentation

The documentation site should give an overview of the CLI.

This is also a good time to add a link to the documentation site ( https://dssg.github.io/triage/ ) from the README

Start by locating the documentation site's current pages and the repository README. Add a concise CLI overview to the documentation and link the documentation site from README; the work is done when both updates are present and the documentation link works.

Written by the indexing model from the issue text.