Docs fix #214014

What does this MR do?

Extend the docs at https://docs.gitlab.com/ee/user/project/deploy_tokens/#gitlab-deploy-token

with

At group level:

• create a deploy token under Settings -> Repository -> Deploy Tokens with read_registry and write_registry
• create a variable under Settings -> CI/CD -> Variables named CI_DEPLOY_USER using the token's Username (protected and masked)
• create a variable under Settings -> CI/CD -> Variables named CI_DEPLOY_PASSWORD using the token's Password/secret (protected and masked)

See the discussion at #214014 (comment 553399691) for details.

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have included a changelog entry.
  • I have not included a changelog entry because _____.
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability 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
• 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