Add DELETE method to the OpenAPI specification (#782)

<!-- Provide a brief summary of your changes -->

## Motivation and Context
<!-- Why is this change needed? What problem does it solve? -->
The following adds a DELETE method to the reference API specification of
the registry.

The endpoint is consistent with the rest of the spec and provides a
standardised way for registry implementations to support deletion if
they choose to.

**Details:**
* Endpoint: DELETE /v0/servers/{serverName}/versions/{version}
* Uses tags: [publish] (same as POST /v0/publish for write operations)
* Requires security: bearerAuth authentication
* Returns ServerResponse on success (200) - matching POST pattern
* Same error responses: 401 (Unauthorized), 403 (Forbidden), 500
(Internal error)
* Added 404 (Not Found) for when server/version doesn't exist
* Parameters match the GET endpoint format with URL-encoding examples
* Marked as "(Optional)" in summary
* Explicit note that it's not implemented by the official registry
* States it's included for standardization purposes



## How Has This Been Tested?
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tested? -->

## Breaking Changes
<!-- Will users need to update their code or configurations? -->

## 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)
- [ ] Documentation update

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

## Additional context
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-->

---------

Signed-off-by: Radoslav Dimitrov <radoslav@stacklok.com>

docs/reference/api/generic-registry-api.md

@@ -18,6 +18,7 @@ The official registry has some more endpoints and restrictions on top of this. S
 - **`GET /v0/servers/{serverName}/versions`** - List all versions of a server
 - **`GET /v0/servers/{serverName}/versions/{version}`** - Get specific version of server. Use the special version `latest` to get the latest version.
 - **`POST /v0/publish`** - Publish new server (optional, registry-specific authentication)
+- **`DELETE /v0/servers/{serverName}/versions/{version}`** - Delete specific server version (optional, not implemented by official registry)
 
 Server names and version strings should be URL-encoded in paths.