chore(docs): add comprehensive triggers documentation

[a7m-1st]

Related Issue

Closes #1444

Description

1. Triggers Overview (docs/triggers/main.md)

   • Explains how triggers work in Eigent
   • Describes the task queue and execution monitoring
   • Overview of all trigger types with use case guidance

2. Schedule Trigger (docs/triggers/schedule.md)

   • Step-by-step setup guide for scheduled workflows
   • Coverage of all schedule types: One Time, Daily, Weekly, Monthly
   • Configuration options including failure thresholds and preview scheduling

3. Slack Trigger (docs/triggers/slack.md)

   • Complete Slack app setup from creation to event subscriptions
   • OAuth scope configuration and permissions
   • Event subscription setup with webhook verification
   • Bot invitation and testing procedures

4. Webhook Trigger (docs/triggers/webhook.md)

   • HTTP request configuration (GET/POST)
   • Security best practices including rate limiting and validation
   • Request filtering with regex and body content matching
   • Integration examples for external services

Testing Evidence (REQUIRED)

• I have included human-verified testing evidence in this PR.
• This PR includes frontend/UI changes, and I attached screenshot(s) or screen recording(s).
• No frontend/UI changes in this PR.

What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

Contribution Guidelines Acknowledgement

• I have read and agree to the Eigent Contribution Guideline

[a7m-1st]

Currently need to test with mintlify dev. Pushing some updates later.