feat: independent versioning for integrations

[nicoloboschi]

Summary

• Integrations now have independent version lifecycles, decoupled from the main Hindsight release
• Per-integration changelog pages added to the docs site at /changelog/integrations/<name>
• New scripts/release-integration.sh <name> <version> script to cut an integration release
• New CI workflow .github/workflows/release-integration.yml triggers on integrations/** tags and publishes to PyPI or npm
• Removed integrations from release.sh and release.yml (no longer bumped or published on main releases)

How to cut an integration release

./scripts/release-integration.sh litellm 0.2.0

This will:

1. Bump the version in the integration's pyproject.toml / package.json
2. Generate a changelog entry via LLM (filtered to commits touching that integration path)
3. Commit version bump + changelog together
4. Create annotated tag integrations/litellm/v0.2.0
5. Push — CI publishes to PyPI/npm automatically

Test plan

• Verify /changelog still resolves correctly (moved to changelog/index.md)
• Verify integration changelog pages render at /changelog/integrations/<name>
• Test release-integration.sh dry-run on a branch
• Confirm release-integration.yml workflow triggers on an integrations/** tag push