Add Triggering Pipelines docs to API readme
What does this MR do?
Adds Triggering Pipelines through the API to the API README page.
However, the Triggering Pipelines definition is a Helper and is not found under /lib/api. I think it is beneficial to have API related documentation referenced here. It would make it more easily navigable for our users.
Customer ticket -> https://gitlab.zendesk.com/agent/tickets/100117 (internal use)
Related issues
Replaces https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/6489 (submitted in EE instead of CE).
Author's checklist
Review checklist
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Technical writer's review (required)
Edited by Evan Read