Docs - Clarify webhook payloads, signatures, and ngrok for receiving webhooks in a development app

[MantisClone]

Problems

• The documentation does not include a complete list of webhook payloads
• An app builder struggled with webhook signatures and ngrok setup

Proposed Solution

• Add a Webhooks page
• Explain webhook payloads
• Explain webhook signatures
• Explain ngrok setup

Considerations

• OpenAPI supports Webhooks, so we should document the webhooks in our OpenAPI spec. Unfortunately, the Nest.js Swagger module does not support webhooks.

[MantisClone]

Nest.js Swagger module unlikely to support anytime soon. Consider alternatives.