Improve documentation

[stephlocke]

• 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
• 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
• 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
• Describe the security best practices used within the solution in docs/security_and_authentication.md
• Write a quickstart guide to getting this solution up and running in docs/getting-started.md
• Have the docs publishing to make a nice docs site