Improve documentation

[ThorstenSauter]

Since the project has grown in complexity, the documentation could use some work. The following items are up for consideration to be added:

• Add Project purpose to documentation #116
• Project branding
• Additional badges (e.g. license information, Snyk vulnerabilites, etc. from shields.io)
• Required configuration to run the API
• Required configuration for running the integration tests
• Infrastructure management #135
• Health checks #134
• Link to a Postman collection for easily importing the available API calls #114
• Links to related documentation (e.g. Microsoft Identity Platform or Azure resource documentation)