Add documentation for RV data products

[awhoward]

Add documentation for all of the RV data products:

• overall RV (was this weighted average of the per-chip RVs implemented?)
• RV per chip
• RVs by order and orderlet
• thorium/LFC/etalon RVs
• simulcal RV

Please update the docs here, specifically the section titled "L2 Primary Extension Header and RV Extension". Note that the link is in the docs-pipeline-info branch. You can edit that branch directly in the Github browser. I suggest adding a mix of text and tables to convey the information, mirroring the information already on the page.

[howardisaacson]

I added some of the header information that was missing on the page linked above. The L0, 2D, L1 header information is now out of date since many more keywords have been added. Do we want to put every header keyword available in that page, or only those that are useful?

Also, much of the information about how to extract RVs from the orderlets is already available on this page:. It doesn't show how to calculate the RV, but it shows how to put every orderlet RV together.