Add doc for scheduled trigger

What does this MR do?

https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/10533 merged into master and will happen in 9.1. This MR is for the documentation.

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #30440 (closed)

@axil Could you review, please? Thanks you!

/cc @ayufan

assigned to @axil

added ~164274 ~15078 ~149423 labels

changed milestone to %9.1

added 1 commit

• 63791bd0 - Update README.md

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added 1 commit

• 379675d4 - Update README.md

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assigned to @dosuken123

added ~19173 and removed ~164274 labels

@dosuken123 I left some comments :)

added 61 commits

• 379675d4...19dd138c - 58 commits from branch gitlab-org:master
• 0d2f06e7 - Add doc for scheduled trigger
• 4f596edb - ok
• a503bdac - Fixed comments

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added 1 commit

• e9ccf22f - Revise botttom notes

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@axil Thank you for reviewing! I resolved all discussions except here. In addition, an explanation for next_run_at is added at bottom. Could you review again, please? Thank you!

mentioned in merge request !10533 (merged)

approved this merge request

resolved all discussions

mentioned in issue #31250 (closed)

merged

mentioned in commit 9d1ce06e

resolved all discussions

Picked into 9-1-stable, will go into 9.1.0-rc6

mentioned in commit c132b29e