feat(providers): add DingTalk (钉钉) notification provider

[ayoubil]

Summary

Adds DingTalk (钉钉) notification provider for sending alerts to DingTalk group chats via webhook.

Closes #6397

What is included

• keep/providers/dingtalk_provider/dingtalk_provider.py — DingTalk webhook provider with optional HMAC sign verification
• docs/providers/documentation/dingtalk-provider.mdx — documentation
• mint.json navigation entry

Provider details

• Type: Notification (outbound)
• Auth: Webhook URL + optional signing secret
• Supports: Text and Markdown message types, @mention all
• Security: Optional HMAC-SHA256 sign verification
• API: https://open.dingtalk.com/document/robots/custom-robot-access

Follows the same webhook pattern as the existing Discord/Slack providers.

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