create docs for horizon command line usage

[tomquisel]

Create a commandline documentation page similar to the one for core.

Who: horizon admins
How: create page that expands upon the commandline help already in the horizon binary and provides examples of common use cases
Why: we want administering Horizon to be easy. This last stellar core outage, we had questions about whether horizon reingest deletes old before reingesting. We had to read the code to find out. Instead, this and other details should be in the docs so admins can handle emergency situations efficiently.