Fix blog post announcement link (#397)

Fix broken blog post announcement link. Currently users are redirected
to the getting started mcp docs.

## Motivation and Context
The existing link redirects to
https://modelcontextprotocol.io/docs/getting-started/intro. The actual
blog post is on the `blog` subdomain.
https://blog.modelcontextprotocol.io/posts/2025-09-08-mcp-registry-preview/

## How Has This Been Tested?
N/A - docs only change

## Breaking Changes
N/A

## Types of changes
<!-- What types of changes does your code introduce? Put an `x` in all
the boxes that apply: -->
- [ ] Bug fix (non-breaking change which fixes an issue)
- [ ] New feature (non-breaking change which adds functionality)
- [ ] Breaking change (fix or feature that would cause existing
functionality to change)
- [x] Documentation update

## Checklist
<!-- Go over all the following points, and put an `x` in all the boxes
that apply. -->
- [x] I have read the [MCP
Documentation](https://modelcontextprotocol.io)
- [x] My code follows the repository's style guidelines
- [x] New and existing tests pass locally
- [x] I have added appropriate error handling
- [x] I have added or updated documentation as needed

## Additional context
<!-- Add any other context, implementation notes, or design decisions
-->

Author: BorenTsai

README.md

@@ -6,7 +6,7 @@ The MCP registry provides MCP clients with a list of MCP servers, like an app st
 
 ## Development Status
 
-**2025-09-08 update**: The registry has launched in preview 🎉 ([announcement blog post](https://modelcontextprotocol.io/blog/registry-preview)). While the system is now more stable, this is still a preview release and breaking changes or data resets may occur. A general availability (GA) release will follow later. We'd love your feedback in [GitHub discussions](https://github.com/modelcontextprotocol/registry/discussions/new?category=ideas) or in the [#registry-dev Discord](https://discord.com/channels/1358869848138059966/1369487942862504016) ([joining details here](https://modelcontextprotocol.io/community/communication)).
+**2025-09-08 update**: The registry has launched in preview 🎉 ([announcement blog post](https://blog.modelcontextprotocol.io/posts/2025-09-08-mcp-registry-preview/)). While the system is now more stable, this is still a preview release and breaking changes or data resets may occur. A general availability (GA) release will follow later. We'd love your feedback in [GitHub discussions](https://github.com/modelcontextprotocol/registry/discussions/new?category=ideas) or in the [#registry-dev Discord](https://discord.com/channels/1358869848138059966/1369487942862504016) ([joining details here](https://modelcontextprotocol.io/community/communication)).
 
 Current key maintainers:
 - **Adam Jones** (Anthropic) [@domdomegg](https://github.com/domdomegg)