Sort list of predefined variables in alphabetical order

What does this MR do?

Sorts in ascending order the list of predefined variables in file doc/ci/variables/README.md The table with the predefined variables shows all the CI_P**** variables between CI_RUNNER and CI_REGISTRY; CI_REGISTRY is after CI_RUNNER.

Replaces Community contribution https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/23805.

Author's checklist

• Apply the correct labels and milestone
• Port the MR to EE (or backport from CE): always recommended, required when the ee-compat-check job fails https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/8861

Review checklist

• Your team's review (required)
• PM's review (recommended, but not a blocker)
• Technical writer's review (required)
• Merge the EE-MR first, CE-MR afterwards https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/8861