Add blog testing instructions to CONTRIBUTING.md (modelcontextprotocol#1437)

* Add blog testing instructions to CONTRIBUTING.md

Added documentation for testing the blog locally, including:
- Link to Hugo installation guide
- Instructions for running the blog server

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[email protected]>

* docs: Fix heading levels in CONTRIBUTING.md

Changed Documentation Guidelines and Specification Proposal Guidelines
from h3 to h2 for better document structure consistency.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[email protected]>

---------

Co-authored-by: Claude <[email protected]>

Author: pja-ant

CONTRIBUTING.md

@@ -69,7 +69,17 @@ npm run check:docs
 npm run format
 ```
 
-### Documentation Guidelines
+## Blog changes
+
+The blog is built using [Hugo](https://gohugo.io/installation/) and located in the [`blog`](./blog) directory.
+
+To preview blog changes locally:
+
+```bash
+npm run serve:blog
+```
+
+## Documentation Guidelines
 
 When contributing to the documentation:
 
@@ -85,9 +95,9 @@ When contributing to the documentation:
 - Follow existing file naming conventions (`kebab-case.mdx`)
 - Include proper frontmatter in MDX files
 
-### Specification Proposal Guidelines
+## Specification Proposal Guidelines
 
-#### Principles of MCP
+### Principles of MCP
 
 1. **Simple + Minimal**: It is much easier to add things to a specification than it is to
    remove them. To maintain simplicity, we keep a high bar for adding new concepts and