Document trigger management on the flow configuration page

What does this MR do?

Documents the trigger controls added to the flow configuration page in !255119 (merged), merged for GitLab 19.5.

Adds a Manage trigger conditions section to doc/user/duo_agent_platform/flows/custom.md, placed after the enable-a-flow content where triggers are already discussed. The section covers:

• Viewing trigger conditions on the flow page, including whether each trigger is on or off.
• Turning a trigger on or off from the flow page.
• Editing a trigger from the flow page through the Trigger actions ellipsis dropdown.
• Deleting a trigger from the flow page through the same dropdown, with the confirmation modal.

The section links to doc/user/duo_agent_platform/triggers/_index.md for full trigger documentation rather than duplicating the trigger creation flow.

Version note: the request behind this MR mentioned GitLab 19.4, but the feature MR merged on September 15, 2026 with milestone 19.5 (and same-day docs MRs use 19.5 in their history entries), so the history entry states GitLab 19.5.

Verified locally with the docs lint hooks: vale, markdownlint, and the documentation link check all pass.

Related issues

• Feature MR: !255119 (merged)

Author's checklist

• Follow the:
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• Optional. Consider taking the GitLab Technical Writing Fundamentals course.
• This MR does not add a new page, so no new product availability details are needed.
• A technical writer review based on the page metadata and the associated Technical Writer is welcome as a non-blocking review.

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