Move usage documentation out of README and into docsite

The documentation should cover the following:

1. Getting started
2. How do I build a job schedule?
3. How do I stop jobs from executing concurrently?
4. How do I schedule a one-off job?
5. How do I name jobs?
6. How many jobs can cardiologist run concurrently?
7. How do I persist my jobs (and why doesn't cardiologist do this for me?)
8. What's the difference between cardiologist and cron?

Start by reading the README and locating the docsite structure. Move the listed usage topics into the docsite, leaving the README focused on project overview and linking to the detailed guidance; done means all eight questions are covered and the README no longer contains the usage documentation.

Written by the indexing model from the issue text.