Clarify behavior of various triggered pipelines

What does this MR do?

From the linked issues, lots of people are struggling with defining rules for rules and only/except, as the values vary depending on the pipeline type, and there are multiple types of "triggered" pipelines:

• api
• pipelines / pipeline
• triggers

This adds lots of clarifications and crosslinks to try to make it clearer which keywords should be used in each situation, and removes some ancient notes spread throughout the triggers docs that made it hard to read.

• Crosslinks in the rules table for pipeline and trigger
• Crosslinks in the only/except table for pipelines and trigger
• Notes in the Multi-project pipelines doc explaining that it's of pipeline/pipelines type for both bridge jobs pipelines and CI_JOB_TOKEN pipelines.
• The triggers doc already had clarification details added in !28442 (merged), so just a light cleanup of the notes.

Related issues

• Closes #28709 (closed)
• Closes #212219 (closed)
• Related to #32414 (closed)

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