Improve documentation for api-manager

[robrap]

1. Lots of the README contents should possible be moved to a docs rST format.
2. @bszabo created (private link) https://2u-internal.atlassian.net/wiki/spaces/TNL/pages/579502286/TNL-10899+API+Gateway to provide api-manager docs for TNL. Much of the contents are not TNL specific, and would probably make more sense as repo documentation useful to anyone. In the meantime, this issue will at least provide a link to find this additional documentation.

[bszabo]

@adzuci @robrap @jmbowman I've created an arch-bom thread to discuss broader issues with regards to getting the API manager to a more usable state. See https://twou.slack.com/archives/C04ACDVM6A1/p1697121746398109