IonPage is Missing from Docs

**URL**
https://ionicframework.com/docs/api

`IonPage` is not documented anywhere. It should probably exist somewhere like:
https://ionicframework.com/docs/api/page

Start at the API documentation index at https://ionicframework.com/docs/api and compare its existing entries with the missing IonPage entry. Add an IonPage page at the proposed https://ionicframework.com/docs/api/page location, ensuring it is discoverable from the API index and documents IonPage.】【。

由索引模型根据 Issue 内容生成。