Improve documentation

- [x] Include goal and architecture
- [ ] Update deployment instructions to have a "Deploy to Azure" button and/or az cli instructions
- [ ] Create documentation for developing the Power BI report
- [x] Add an intro to the application modernisation tools being used in `docs/appcat.md`
- [ ] Describe the architecture and design decisions made in `docs/architecture.md`
- [x] Write a how-to guide for developing Functions locally (balance single place of info with relying on Learn docs to ensure info is up to date) in `docs/developing_functions_locally.md`
- [x] Describe the security best practices used within the solution in `docs/security_and_authentication.md`
- [x] Write a quickstart guide to getting this solution up and running in `docs/getting-started.md`
- [x] Have the docs publishing to make a nice docs site