Update triggers documentation for card layout

What does this MR do?

Updates the triggers documentation to match the card-based layout introduced in !255035 (merged).

Changes:

• Turn a trigger on or off: the step referenced the Status column, which no longer exists. The toggle now sits on the right side of each trigger card, and inactive cards are visually subdued.
• Edit a trigger: the step referenced an Edit flow trigger pencil icon button. The edit action now lives in the More actions (vertical ellipsis) menu on the trigger card as Edit trigger.
• Delete a trigger: the step referenced a Delete flow trigger remove icon button. The delete action now lives in the More actions menu on the trigger card as Delete trigger.
• Adds a history entry for the table-to-cards layout change in GitLab 19.5.

UI wording verified against ee/app/assets/javascripts/ai/duo_agents_platform/pages/flow_triggers/index/components/flow_trigger_card.vue.

Related issues

• Relates to !255035 (merged)

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