Docs Feedback: Detail how to passthrough variables in Parent-Child pipelines

Problem to solve

With the new Parent-child pipelines it's not clear how to pass through variables from the parent to the child in the docs. This functionality is present though and working but it's detailed in a different section on the Multi-Project pipelines page.

Docs should be updated on the Parent-child pipelines page to show users how to do this also.

Proposal

Bit awkward as we typically want to be DRY but the pages are separate so it may just need to be the same section.

Other links/references

Issue from user asking for the feature due to docs not being clear #213729 (closed)