Create draft documents on creating Entra sign in credentials

https://gitlab.developers.cam.ac.uk/uis/devops/experiments/azure-application-factory/ contains a prototype application credentials factory for Azure Entra application sign in credentials. Create some sample docs for this which outline how sign in may be added to applications.

Use these docs as a framework to build a productised service around. DO NOT MERGE THE DOCS until the service is launched.