Establish tagged GitHub Releases with versioned asset downloads

## Problem

There is currently no stable, versioned download URL for any release of the VDM standard. This makes it impossible for tooling to:

- Programmatically fetch a specific version
- Verify integrity against a known checksum
- Automate dependency updates

VSS sets the right precedent with tagged releases on GitHub:
```
https://github.com/COVESA/vehicle_signal_specification/releases/download/v4.0/vss_rel_4.0.yaml
```

## Proposed action

1. **Create a tagged release** — tag `v1.1` (or the current stable version), attach the canonical YAML/JSON file as a release asset. The asset URL would be e.g.:
```
https://github.com/COVESA/vdm/releases/download/v1.1/vdm-1.1.yaml
```
2. **Document the pattern** — add a note to `CONTRIBUTING.md` (or create one):
> When a new version of this standard is published, create a tagged GitHub Release (vX.Y) and attach the canonical machine-readable file as a release asset. This allows tooling to fetch and verify specific versions programmatically.

## Follow-up

Once a release exists, VISSR's `feat/s2dm-ci` workflow can reference a stable URL instead of `HEAD`. Other COVESA tooling (s2dm, vss-tools) can also pin to specific VDM versions.

Happy to open a draft PR with the CONTRIBUTING.md change if that's helpful.