Add email observability topic for monitoring email delivery via SendGrid Overwatch API

[catplat]

Summary

Add a new documentation topic covering the SendGrid Overwatch email observability API, which lets users query delivery logs, statistics, and suppression lists for emails sent from their Platform.sh environments.

Changes needed

• Add sites/platform/src/development/email-observability.md: New topic covering authentication, message history (by date, recipient, and event type), delivery statistics, and suppression list management (bounces and blocks)
• Add email-observability to sidebar navigation after the existing email entry

Notes

• Adapted from the corresponding Upsun devcenter PR: https://github.com/upsun/devcenter-mintlify/pull/405
• The base URL https://platform.sendgrid.pltfrm.sh needs confirmation — verify this is the correct endpoint for Platform.sh