Add warning and FAQ for upcoming WhatsApp integration provider change

[alexrelevanceai]

Summary

Our WhatsApp integration provider, Unipile, is shipping a change to their integration in mid-March 2026. This PR updates the WhatsApp Personal integration docs page to inform users about the upcoming changes.

Changes made

1. Warning banner at the top of the page — A prominent <Warning> callout has been added above the "Connect the integration" section, alerting existing users that:

   • They will need to reconnect their WhatsApp account once the provider change takes effect.
   • Previously threaded conversations will start on a new thread, as the underlying identifier format is changing.
   • Affected users have already been emailed with further details.

2. New FAQ entry — Added a new accordion item titled "I received an email about a change to the WhatsApp integration. What do I need to do?" to the FAQs section, providing a clear answer for users who arrive at the docs after receiving the notification email.

Context

• Slack thread: https://qualitativecloud.slack.com/archives/C0A9Z7N6Q2E/p1772579581591059
• This only affects users who are currently using the WhatsApp Personal integration.
• No action is required from users until the change goes live — follow-up communications will provide further instructions.

Test plan

• Verify the warning banner renders correctly on the WhatsApp integration docs page
• Verify the new FAQ accordion expands and displays the correct content
• Confirm no formatting or rendering issues on the page

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