Add documentation for csv selectors

[brosenberg42]

Documentation for: openmpf/openmpf#1966

Issues:

• Document CSV media selectors openmpf#1981

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This change is 

[jrobble]

@jrobble reviewed 5 of 5 files at r1, all commit messages.
Reviewable status: all files reviewed, 1 unresolved discussion (waiting on @brosenberg42)

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a discussion (no related file):
Overall, good job and great explanation of encoding issues.

I simplified some things and performed some cleanup for clarity in this branch: https://github.com/openmpf/openmpf.github.io/blob/jrobble/csv-selector/docs/docs/Media-Selectors-Guide.md

Please review and merge that into this one.

I added some TODOs in there for you. The doc was focusing heavily on JSON_PATH and needs complete CSV_COLS for clarity and simplicity. Also, I didn't like how I had to scroll all the way past the CSV_COLS stuff to find the example JSON_PATH output. I think it's better if the JSON_PATH and CSV_COLS sections are more self-contained - although I think it's fine that the "New Job Request Fields" section is common and focuses on JSON_PATH since we really don't need to explain those fields twice. It should be clear from the CSV_COLS example request JSON (that I asked you to add) how they're being used for CSV.

[jrobble]

@jrobble reviewed 5 files and all commit messages, and resolved 1 discussion.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @brosenberg42).