Add a Gotenberg capture driver - #7

Summary

I already use Gotenberg in my projects, so installing Chrome locally adds browser binaries and system dependencies to the application image. This driver lets applications reuse an existing Gotenberg service for screenshots and PDFs while keeping the same fluent API.

Screenshot::driver('gotenberg')
    ->html($html)
    ->javascript()
    ->pdf()
    ->capture();

Evidence

• Before: The built-in capture driver required Chrome on the application host.
  After: PNG, JPEG, and PDF captures pass integration tests against Gotenberg, and CI passes.

Merge Danger

Door: Two-way

Blast Radius: Captures

The new driver is opt-in. Gotenberg-specific limitations and optional GD cropping are documented.