Security and effects 'inner workings'

[roberth]

See https://github.com/hercules-ci/hercules-ci-effects/issues/31

• [ ] New page describing security model.
• [ ] A description of how the effects sandbox works, including secrets handling, environment variables, etc. End goal: reader should be able to write the various "low-level" hercules-ci-effects themselves. Ie a complete reference.
• [ ] Anything else I've mentioned in the issue