usgscsm for GXP (transfer WIKI to docs)

[thareUSGS]

Documentation Topic
CSM camera model information, generally for SOCET GXP

Description of Task
Transfer and update of information from original usgscsm wiki to asc docs.
https://github.com/DOI-USGS/usgscsm/wiki

[thareUSGS]

I can help with this but currently can't assign anyone (including myself).