Working on documentation

[jzonthemtn]

Work in progress but hopefully a start. Moved to a top-level documentation.md for visibility. I brought in @RasonJ's mappings docs and replaced the screenshot of the mappings with links to the mapping files in case the screen shot gets outdated.

[RasonJ]

The purpose of the image is have the user focus on the main fields and not get lost in json ickiness. I'd prefer to have a collapsable display highlighting the primary fields, but without that, I think we need an image, at least while we're talking people through the schemas.

[epugh]

Could we be more elaborate in the mark down? I noticed that they do this kind of pattern: https://opensearch.org/docs/latest/query-dsl/term/term/#parameters to demonstrate a JSON based API, and then below the formatting...