Improve documentation for API app

[tanya-borisova]

Improve docs under https://microsoft.github.io/AzureTRE/tre-developers/api/

• Do we still need to follow the manual steps as there's now a debug target in devcontainer doing it for us?
• If we still need it, the manual steps should be moved to a script
• Text around implementation details (Auth, Network requirements) should either go to a separate page or go away, as it distracts from setting up development with API app
• Missing instructions on how to debug an instance deployed to the cloud
• Explain how to work with Swagger UIs (different UIs for workspaces, Authorize button)