docs - Sort predefined variables in ascending order

What does this MR do?

I sorted 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.

Related issues

No issue related

Author's checklist

No content modification. Just sort order. Do not need to check or crosslink documentation.

• Apply the correct labels and milestone
• Crosslink the document from the higher-level index
• Crosslink the document from other subject-related docs
• Feature moving tiers? Make sure the change is also reflected in features.yml
• Correctly apply the product badges and tiers
• Port the MR to EE (or backport from CE): always recommended, required when the ee-compat-check job fails

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