[Docs] Add documentation for manually testing Fleet Server API endpoints

Fleet Server's API endpoints (e.g. check in) require an active agent API key and some knowledge of how to provide that API key. We should document the process by which API endpoints can be manually tested either through `cURL` or an API client (e.g. Postman, Thunder Client, Insomnia, etc) so developers can sanity check updates to API endpoints outside of just unit/integration tests.

These docs should be added to the README.