docs(migration): add missing notifications.md for MiniKit to Farcaster

[faisalnugroho]

What

Adds the missing notifications.md file that mapping.md references for server-side notification implementation during MiniKit to Farcaster migration.

The problem

mapping.md references NOTIFICATIONS.md in two places (lines 412 and 452):

See [NOTIFICATIONS.md](NOTIFICATIONS.md) for server-side implementation.

But that file doesn't exist. Anyone following the migration guide for the notification section hits a dead link and has to figure out the Farcaster notification API on their own.

What's in the new file

Created notifications.md with:

1. Overview — explains the shift from client-side (MiniKit) to server-side (Farcaster SDK) notifications
2. Webhook registration — how to register your app's notification endpoint
3. Token storage — how to store user notification tokens when they opt in
4. Server-side send — complete example of sending notifications via the Farcaster API, including handling expired tokens
5. Client-side trigger — minimal client code that calls your server endpoint
6. Comparison table — key differences between MiniKit and Farcaster SDK notification patterns
7. Security notes — API key handling, rate limiting, token validation

Also fixed the link casing (NOTIFICATIONS.md → notifications.md) to match the actual filename.

Fixes #30

[cb-heimdall]

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[faisalnugroho]

Hi! Following up on this docs PR. This PR adds the missing notifications.md migration doc for the MiniKit to Base App transition, fixing a broken internal reference in the migration guide.

All CI checks pass (StepSecurity ✅). Would love to get your review when you have a moment. Thanks!

[faisalnugroho]

Hi! I'd love to get this PR reviewed when you have a moment. This PR addresses add missing notifications.md for MiniKit to Farcaster migration. Happy to make any adjustments based on your feedback. Thank you!