[Feature Request] Add available Regions to the Namespace Resource documentation

[jlacefie]

Is your feature request related to a problem? Please describe.

To help users configure regions correctly through the TF provider, it's recommended to add, or link, the list of available regions in the TF docs.

Describe the solution you'd like

Add available regions to the Namespace Datasource docs

[jlacefie]

link the regions documentation from Temporal to the TF Provider as a reference.

[jlacefie]

Regions are documented here