More documentation rework
- Internals/How CI/CD integrations works: add GitHub Actions article.
- Reference/werf.yaml: small improvements
- Fix all broken links.
- Actualized README.
- Small fixes for introduction, using-with-ci-cd-systems, quickstart and documentation/overview articles.
Fixes
- [dockerfile] Fix FROM with composite ARG variable value is not resolved.
- [ci-env] Fix extra annotation with workflow run url for GitHub Actions.