[Chainhook] Document chainhook trigger payload

[MicaiahReid]

When a chainhook is triggered, we don't have any documentation on the payload.

This was requested in a discord thread: https://discord.com/channels/621759717756370964/1222070000886616064

[ryanwaits]

@MicaiahReid noted: this will be added to new docs revamp

[ryanwaits]

ref: https://github.com/hirosystems/docs/pull/724