Introduce Goreleaser (#366)

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

## Motivation and Context
<!-- Why is this change needed? What problem does it solve? -->
The following PR introduces Goreleaser for the releases of the registry
server and the publisher CLI.

**Details:**
- Builds registry and publisher binaries for Linux, macOS, and Windows
(amd64 + arm64)
- Leverage GitHub's release process by creating releases directly from
GitHub's interface
- Added .goreleaser.yaml configuration
- New .github/workflows/release.yml workflow triggered by GitHub
releases
- Updated Docker tagging in deploy workflow (:latest and :semver(the
tag) for releases, :main-YYYYMMDD-sha for testing/development)
- Consolidated version handling in main.go files
- Added release documentation

## How Has This Been Tested?
<!-- Have you tested this in a real application? Which scenarios were
tested? -->

Locally (but we have to test it by creating a release, i.e. v0.0.1 once
this gets merged)

## Breaking Changes
<!-- Will users need to update their code or configurations? -->
Yes, the notion of `latest` for the container image is now different

## 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)
- [x] 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
<!-- Add any other context, implementation notes, or design decisions
-->
Related to #358

---------

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

docs/releasing.md

@@ -0,0 +1,27 @@
+# Release Guide
+
+## Creating a Release
+
+1. **Go to GitHub**: Navigate to https://github.com/modelcontextprotocol/registry/releases
+2. **Click "Draft a new release"**
+3. **Choose a tag**: Click "Choose a tag" and type a new semantic version that follows the last one available (e.g., `v1.0.0`)
+5. **Generate notes**: Click "Generate release notes" to auto-populate the name and description
+6. **Publish**: Click "Publish release"
+
+The release workflow will automatically:
+- Build binaries for 6 platforms (Linux, macOS, Windows × amd64, arm64)
+- Create and push Docker images with `:latest` and `:vX.Y.Z` tags
+- Attach all artifacts to the GitHub release
+- Generate checksums and signatures
+
+## After Release
+
+- Docker images will be available at `ghcr.io/modelcontextprotocol/registry:latest`
+- Binaries can be downloaded from the GitHub release page
+
+## Versioning
+
+We use semantic versioning (SemVer):
+- `v1.0.0` - Major release with breaking changes
+- `v1.1.0` - Minor release with new features
+- `v1.0.1` - Patch release with bug fixes