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>

README.md

@@ -67,14 +67,23 @@ The service runs on [`localhost:8080`](http://localhost:8080) by default. This c
 Pre-built Docker images are automatically published to GitHub Container Registry:
 
 ```bash
-# Run latest from main branch
+# Run latest stable release
 docker run -p 8080:8080 ghcr.io/modelcontextprotocol/registry:latest
 
-# Run specific commit build
-docker run -p 8080:8080 ghcr.io/modelcontextprotocol/registry:main-20250806-a1b2c3d
+# Run latest from main branch (continuous deployment)
+docker run -p 8080:8080 ghcr.io/modelcontextprotocol/registry:main
+
+# Run specific release version
+docker run -p 8080:8080 ghcr.io/modelcontextprotocol/registry:v1.0.0
+
+# Run development build from main branch
+docker run -p 8080:8080 ghcr.io/modelcontextprotocol/registry:main-20250906-abc123d
 ```
 
-**Available tags:** `latest`, `main-<date>-<sha>`
+**Available tags:** 
+- **Releases**: `latest`, `v1.0.0`, `v1.1.0`, etc.
+- **Continuous**: `main` (latest main branch build)
+- **Development**: `main-<date>-<sha>` (specific commit builds)
 
 </details>
 
@@ -87,7 +96,7 @@ To publish a server, we've built a simple CLI. You can use it with:
 make publisher
 
 # Use it!
-./cmd/publisher/bin/mcp-publisher --help
+./bin/mcp-publisher --help
 ```
 
 See [the publisher guide](./docs/guides/publishing/publish-server.md) for more details.