Add docs on post-merge pipelines

What does this MR do?

Add docs on post-merge pipelines for https://gitlab.com/gitlab-org/gitlab-ce/issues/25140.

Related issues

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/53527

Author's checklist

• Apply the correct labels and milestone
• Crosslink the document from other subject-related docs
• 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