Documentation: `page:index` event

[tomodachi94]

This event has been referenced at least twice in the forums in the past few weeks as an answer to a question:

• https://community.silverbullet.md/t/how-to-extend-index-from-hooks/3060/5
• https://community.silverbullet.md/t/how-to-create-a-object/3035/2

However, there's no documentation for this event in the event documentation.