Add developer docs for outbound webhooks

[courier-new]

We've added user-facing documentation for outbound webhooks for site admins to understand how to take advantage of them for Batch Changes events. Now, we should add internal developer documentation for understanding how the outbound webhooks system works and how to expand it to other areas of the product. We should consider as part of this work how we will encourage adoption by other teams and "graduate" outbound webhooks from being owned by Batch Changes.

Docs should include:

• Technical overview of the implementation (packages, DB schema overview, background worker, etc.)
• How to add new event types with code examples
• Future plans for scoping to entities

[github-actions[bot]]

Hey, @sourcegraph/batchers (@eseliger @courier-new @adeola-ak @BolajiOlajide @Piszmog @malomarrec @chrispine @danielmarquespt) - we have been mentioned. Let's take a look.