Establish tagged GitHub Releases with versioned asset downloads

## Problem

There is currently no stable, versioned download URL for any release of the S2DM 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 `v0.1` (or the current stable version), attach the canonical schema file(s) as release assets. The asset URL would be e.g.:
```
https://github.com/COVESA/s2dm/releases/download/v0.1/s2dm-0.1.schema.json
```
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.

## Motivation

VISSR's `feat/s2dm-ci` workflow (`uvx covesa-s2dm validate`) currently pulls the latest S2DM version on every CI run. A tagged release would allow CI workflows to pin a specific validated version and detect drift when the standard updates.

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