Triggers docs

What does this MR do?

Introduces missing documentation for updated pipeline triggers:

• https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/9713
• https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/9277

Does this MR meet the acceptance criteria?

• Documentation created/updated

added 3 commits

• 1726cc03...7a623907 - 2 commits from branch master
• 6cf8d343 - Update triggers documentation

Compare with previous version

changed milestone to %9.0

added ~149423 label

approved this merge request

enabled an automatic merge when the pipeline for 6cf8d343 succeeds

canceled the automatic merge

merged

mentioned in commit 40e3a701

mentioned in issue #29872 (closed)

Picked into 9-0-stable and will go out in 9.0.1

removed ~149423 label