Add info on CI/CD variable encryption

What does this MR do?

Adds info about how CI/CD variables are encrypted to the docs.

Slack discussion: https://gitlab.slack.com/archives/C0AR2KW4B/p1583161080371800

we have a customer that is interested in understanding where masked variables are stored for GitLab,com (assuming self-managed will be in their database). I don't see any explanation for how secrets are stored or levels of encryption.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team