Add page to documentation regarding CI/CD security

[legetz]

In the case of dev major branch in CI/CD, Hardis expects to see config/branches/.jwt/dev.key file which is used for authentication towards sandbox. File is encrypted and decrypted on-the-fly with secure CI/CD environment variables but would be good to open up this angle in documentation to keep security-minded people happy.

Not urgent issue but at some point handy addition to documentation.