Add a man page for below

Summary:
This is a man page for `below`.
This is written from the various `below * --help` output.
Some sections were written by converting the help output with:
```
awk '/* Example Commands */{exit}{if(p)print}/* Aggregated fields */{p=1}' |
   sed -e 's/^\* /\\fB/' -e 's/:/\\fR:/'
```

Reviewed By: lnyng

Differential Revision: D67611255

fbshipit-source-id: 4aa05ae930c8f80c62bcfa24db6b85c8ad29055f