Improve entry point documentation to SDK generation from API spec

Now that our users are using Typespec, we need to guide them on documentation to configure the emitters in their projects.
The current entry point is: https://github.com/Azure/azure-rest-api-specs/blob/main/documentation/Getting-started-with-TypeSpec-specifications.md#arm which doesn't explain too much.

Things to add:
- How to configure the emitters
- Which ones
- Who can help?
- Why is it needed

Should we move this documentation to EngHub too?

Start with documentation/Getting-started-with-TypeSpec-specifications.md and its ARM section, linked in the issue. Review how the entry point currently describes TypeSpec and identify the documentation needed for emitter configuration, available emitters, support contacts, and why configuration is required. Done means the entry point covers those topics and the EngHub relocation question is resolved.

Written by the indexing model from the issue text.