improve documentation of reports

[wholmgren]

The report documentation should include

• Considerations around specifying probabilistic reports
• What to expect in a deterministic report, an event report, and a probabilistic report
• The list under "following these steps" should bold the action sections.
• Clarify the process around sites vs aggregates

I also want to consider moving the report subsection from the "working with data" section into its own top level section. Maybe add some examples.

(I couldn't remember the status of the event reports, so I tried to look it up in the documentation.)