Document design decision for module design

## Description

We have arbitrarily said that we have chosen to "stay close to Azure Resource Reference" without really defining what we mean by this. We should have a wiki page under module design or so where we declare the module design decision we have made for general module development.

```[tasklist]
### Tasks
- [ ] [Wiki module design] Elaborate on why we add optional features such as rbac and diagnostic settings
- [ ] [Wiki module design] ARR alignment - Documentation link in readme
- [ ] [Wiki module design] ARR alignment - Folder structure alignment
- [ ] Azure/bicep-registry-modules#2509
- [ ] [Wiki module design] General design principles - Resource names in main Bicep template
- [ ] [Wiki module design] Test cases - detail why we are using resources instead of modules when deploying dependencies
```