Add basic docs about cross-project pipeline triggers

What does this MR do?

Docs for https://gitlab.com/gitlab-org/gitlab-ee/issues/8997

Related issues

https://gitlab.com/gitlab-org/gitlab-ee/issues/8997

Author's checklist

• 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

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