[Task] Add Region Support Explainer

### Description

Right now, the default settings for this repo's resources include very specific regions due to dependency requirements between Azure's AI resources and Power Platform's primary and failover VNets.

The reasoning and Microsoft documentation behind these choices should be made explicit in the documentation. For example, we should have a reference to [Power Platform's supported VNet regions](https://learn.microsoft.com/en-us/power-platform/admin/vnet-support-overview#supported-regions) for users to consider.

### Definition of Done

```markdown
- A user of this repo can review region dependencies and possible conflicts by starting with documentation in this repo.
```

### Values Provided

Users will be empowered to update region properties to better fit their needs without worrying about resources breaking or becoming incompatible.