Use 'latest' tag for mcp-publisher downloads (#610)

## Summary

This PR removes version numbers from release asset names and updates
documentation to use GitHub's `latest` tag for mcp-publisher downloads.

## Changes

1. **Updated `.goreleaser.yaml`**: Removed `{{ .Version }}` from asset
name templates
   - Changed from: `mcp-publisher_{{ .Version }}_{{ .Os }}_{{ .Arch }}`
   - Changed to: `mcp-publisher_{{ .Os }}_{{ .Arch }}`

2. **Updated documentation**: Changed all download URLs to use `latest`
tag instead of version-pinned URLs
   - `docs/guides/publishing/publish-server.md`
   - `docs/guides/publishing/github-actions.md`

## Benefits

- Documentation stays automatically up-to-date without manual changes or
workflows
- Users always get the latest version by default
- Version pinning still possible using specific tags (e.g.,
`https://github.com/modelcontextprotocol/registry/releases/download/v1.2.3/mcp-publisher_darwin_amd64.tar.gz`)
- Simpler maintenance and better user experience

## Notes

The new asset naming will take effect with the next release. After that,
users can immediately start using the `latest` tag.

Fixes #607

.goreleaser.yaml

@@ -120,12 +120,12 @@ signs:
 archives:
   # Registry server archive
   - id: registry
-    name_template: "registry_{{ .Version }}_{{ .Os }}_{{ .Arch }}"
+    name_template: "registry_{{ .Os }}_{{ .Arch }}"
     ids:
       - registry
-  
+
   # Publisher CLI archive
   - id: publisher
-    name_template: "mcp-publisher_{{ .Version }}_{{ .Os }}_{{ .Arch }}"
+    name_template: "mcp-publisher_{{ .Os }}_{{ .Arch }}"
     ids:
       - publisher