docs: add new webhook event types

[golobitch]

Due to the #3012 we need to add two new webhook event types to the documentation

Asset Liquidity High

Value: asset.liquidity_high
Description: Asset liquidity is above defined threshold

Peer Liquidity High

Value: peer.liquidity_high
Description: Peer liquidity is above defined threshold

This is ofcourse just a sample text.

[mkurapov]

@golobitch thanks for adding this so we don't forget to update the docs, I'm going to add the user-docs label and let @melissahenderson handle the priority

[melissahenderson]

Adding a note to not forget to update resources/webhook-event-types.