Move Platform pages to a top level menu item

[kbaley]

The Service Platform pages are buried under the NServiceBus menu item when they talk about the entire platform. They would be better served as a top-level menu item (possibly above NServiceBus and below Architecture.

Besides the page above and all its subpages, consider moving these pages under the same top-level heading:

• https://docs.particular.net/platform/contributing
• https://docs.particular.net/nservicebus/release-notifications

The new top level menu item will need an icon, probably the one associated with the Particular Service Platform.