Document expectations for scheduler running jobs

[JasonWeill]

Problem

Based solely on the docs, it's not clear if a user's scheduled jobs will continue to run after they, for example, log out.

Proposed Solution

Update the documentation to clearly indicate that if a user logs out or quits Jupyter Server, their scheduled jobs will not run.

Additional context

We may have to update documented expectations after implementing #113.

[JasonWeill]

This could be two separate docs, per discussion with @dlqqq:

1. A "How does the scheduler work?" doc to explain, at a high level, the way that the scheduler runs jobs while the server is running
2. A troubleshooting page to help users diagnose why their scheduled jobs didn't run as expected.