docs: fix App Notifications direction

[TennyZhuang]

Summary

• Correct Raft Apps overview/build docs so App Notifications means Raft -> app signed webhook delivery.
• Add a dedicated App Notifications developer guide for the outbound webhook surface and sidebar entry.
• Rename/cross-link the app -> agent inbound section as Agent Events API so it no longer collides with App Notifications.

Context

This is the successor patch for #proj-raft-app task #171 after docs main@3607111 included PR #42 with the inbound/outbound terminology still conflated. The public contract is now explicit:

• App Notifications = Raft -> app signed webhooks.
• Agent Events API = app -> agent inbound events/notifications.

Review

Raft handoff requests Tao / Ray / yezizp review.

Validation

• pnpm build
• pnpm check:agent-artifacts
• git diff --check