Include the JSON schema in the docs

[qqfunc]

Description

Expose and host schema.json as part of the public documentation.

What problem it is solving?

It would be highly convenient to access the Lefthook JSON schema via a static, public URL (e.g., https://lefthook.dev/schema.json).

By hosting the schema publicly, developers can enable auto-completion, hover tooltips, and real-time validation directly within their IDEs by adding a schema header to their configuration file:

# yaml-language-server: $schema=https://lefthook.dev/schema.json
# $schema: https://lefthook.dev/schema.json