Update docs for multi-project pipelines

Alexandr Tanaynorequested to merge
docs-multiproject-pipelines-tiers into master

What does this MR do?

EDIT: From Marcel: I have updated the contribution substantially, as there was more involved than expected, see !26560 (comment 299474635)

Closes #209041 (closed)

Previous:

The feature Multi-project pipelines but some important parts of its functionality are available in core:

This MR adds Core labels to both sub-sections to make it clear.

cc @marcel.amirault

Edited by 🤖 GitLab Bot 🤖

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