Docs reorg

Summary Docs Reorg Part 1 of N

This isn't done done, but I want to avoid a water fall merge, and do the doc reorg over time.
This is at a good enough checkpoint that it should be reviewable & mergeable.
The remaining reorg of .md files in the /docs folder can be done over time
(I'd like to make it so only a README.md exists there and use it as the primary entry point.)

Use this to preview: https://repo1.dso.mil/platform-one/big-bang/bigbang/-/tree/doc-reorg/docs

Summary of Changes:

1. Added a Main README.md
2. Renamed /docs/understanding to /docs/understanding_bigbang
3. Added a /guides/deployment_scenarios/quickstart.md as scaffolding to help others get idea of where to org docs in future
4. Moved contents of /docs/d_prerequisites.md to /docs/guides/prerequisites
5. Deleted /docs/c_faqs.md that was empty
6. Added /docs/understanding_bigbang/README.md with Useful Background Contextual Info
7. Jon added Flux install docs from the cohort
8. Added further elaboration on OS, kube distro, and storage class prequisites, and created a table to clarify and set expectations that different deployment scenarios result in different prerequisites.

Hot Spots I know I can use feedback on:

1. (.md links could use testing as well)
2. (the prereqs table)