Add namespace approval process documentation to spec repo wiki

## Summary

Add a wiki entry for the Namespace Approval process so that the spec repo chatbot and developers can discover how the check works.

## Context

The namespace approval workflow ([PR #44085](https://github.com/Azure/azure-rest-api-specs/pull/44085)) adds a new required check. Other checks are documented in the [spec repo wiki](https://github.com/Azure/azure-rest-api-specs/wiki), which helps service teams understand what blocks their PR and how to resolve it.

## What to add

- A wiki page explaining the `Namespace Approval` check
- What triggers it (tspconfig.yaml changes with emitter options)
- How to get approval (architect applies label)
- Who the approvers are per language
- Link to the process doc: `.github/workflows/src/namespace-approval/NAMESPACE-REVIEW-PROCESS.md`

## Related

- PR: https://github.com/Azure/azure-rest-api-specs/pull/44085
- Review comment: https://github.com/Azure/azure-rest-api-specs/pull/44085#discussion_r3478779866

Start by reading .github/workflows/src/namespace-approval/NAMESPACE-REVIEW-PROCESS.md and reviewing the existing spec repo wiki check pages for structure and terminology. Add a wiki page covering the Namespace Approval trigger, approval label, language-specific approvers, and process-document link. Done means developers and the spec repo chatbot can find and understand how to resolve the check.

Written by the indexing model from the issue text.