Delivery system: document the webhook/event system

[nntin-bot]

Part of #87.

Write docs/webhooks.md explaining the webhook/event delivery system this feature adds — the reader is a third-party integrator (e.g. whoever builds the Pico Discord Bot side, per #87's own explicit "Scope" section: "Providing the interface and the instruction for the Discord Bot as a GitHub comment in this issue is in scope").

Cover, in plain terms before any schema detail (schema itself is #89, not this one):

• What triggers an event: an authenticated user hitting "share" on a layout.
• The rate limits on triggering a share (1 request / 5 minutes, max 5 / day) and why they exist.
• How a service subscribes: a moderator-issued secret, tied to a Discord user id, created via a page presented to moderators.
• What the admin page shows moderators about existing subscriptions (who created what).
• At a high level, what's delivered and how a subscriber is expected to consume it (this doc links to the schema doc for the exact shape rather than duplicating it).
• The end-to-end example from Add webhook delivery system #87's own goal: a user hits share, the Pico Discord Bot (subscribed) fetches the preview image and posts it to Discord with the sharer's Discord user id.

This documents the SYSTEM as designed/being built — write it alongside or slightly ahead of the implementation issue (#91) so the implementation has a clear target to build against, and update it if reality diverges during implementation.

Branch: open the PR against feat/delivery-system (already branched off develop), not develop directly. feat/delivery-system merges into develop as its own PR once all of #87's sub-issues land.