docs: add full omnibus example for smime signing

What does this MR do?

• Improve Omnibus documentation for SMIME signing
• Document only RSA keys supported
• Minor style

Triggered by: omnibus-gitlab!3514 (comment 206968048)

The development of this MR is sponsored by siemens (/cc @bufferoverflow).

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry for user-facing changes, or community contribution. Check the link for other scenarios. (not applicable)
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

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