Properly sort CI_MERGE_REQUEST_ID in variables doc
doc: Properly sort CI_MERGE_REQUEST_ID
The CI_MERGE_REQUEST_ID variable is not properly sorted, moving it away from CI_MERGE_REQUEST_IID, which is confusing for lazy readers. Lets fix this to make the documentation more consistent by grouping them together.
This MR does not change any code or text, but moves a table entry.