Proven approaches and recipes for most popular CI systems

[distorhead]

• GitLab CI support has been implemented a while ago (and recently updated via #2391, #2416, etc.). The ready-to-use guide is published in documentation.
• GitHub Actions support has been recently implemented in #2210. The guide is here.
• An issue for Jenkins & TeamCity is #1150.
• Here is a general overview of how you can integrate werf with other CI systems.

[distorhead]

We have decided, that a good general instruction about embedding werf into any CI/CD system is enough. We tried to write such instructions:

1. https://werf.io/documentation/using_with_ci_cd_systems.html
2. https://werf.io/documentation/advanced/ci_cd/generic_ci_cd_integration.html
3. https://werf.io/documentation/internals/how_ci_cd_integration_works/general_overview.html