Document slicer_cli_web

[cooperlab]

Create documentation for an example algorithm being wrapped for HistomicsUI using slicer_cli_web, and integrate this into existing documentation.

[manthey]

We have some better samples now on slicer_cli_web (e.g., https://github.com/girder/slicer_cli_web/tree/master/example-average-color).

[manthey]

Closing this. A separate issue might be to make a cookiecutter/wizard to wrap an algorithm somehow.