feat: Add website_url field for MCP servers (#422)

Adds optional website_url field to the server.json schema allowing
publishers to provide a url to the server's homepage, documentation, or
project website. This provides a central link for users to learn more
about the server. Particularly useful when the server has custom
installation instructions or setup requirements.

Issue: #183 
PR: #130 
Discussion: #274 

## Motivation and Context
This change addresses the need identified in #130, #183 and #274 for a
standardized way to direct users to an official landing page similar to
`homepage` used in NPM
([example](https://www.npmjs.com/package/@modelcontextprotocol/sdk)). It
is also serves as a viable option for MCP Servers that don't follow
typical package manager patterns (e.g., embedded in a Desktop app).

## How Has This Been Tested?
- Schema validation: All 13 example server.json files validate
successfully against the updated schema
- Go validator tests: Added additional test cases covering URL format
validation and namespace matching
- Run all checks: Updated example files to work with anonymous
publishing and verified `make check` passes

## Breaking Changes
None. The website_url field is optional and backward compatible with
existing server.json files.

## Types of changes
- [ ] Bug fix (non-breaking change which fixes an issue)
- [x] New feature (non-breaking change which adds functionality)
- [ ] Breaking change (fix or feature that would cause existing
functionality to change)
- [x] Documentation update

## Checklist
- [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
Security considerations:
- Website URL validation enforces that the domain matches the
publisher's reverse-DNS namespace
- HTTP/HTTPS protocols are required (no file:// or other schemes)
- Validation is consistent with existing remote URL validation patterns

docs/reference/api/openapi.yaml

@@ -233,6 +233,11 @@ components:
           type: string
           example: "1.0.2"
           description: "Version string for this server. SHOULD follow semantic versioning (e.g., '1.0.2', '2.1.0-alpha'). Equivalent of Implementation.version in MCP specification."
+        website_url:
+          type: string
+          format: uri
+          description: "Optional URL to the server's homepage, documentation, or project website. This provides a central link for users to learn more about the server. Particularly useful when the server has custom installation instructions or setup requirements."
+          example: "https://modelcontextprotocol.io/examples"
         created_at:
           type: string
           format: date-time