docs(examples): document external compute driver authoring and extensibility

Description

Publish an external compute-driver authoring guide that explains how third parties implement, run, configure, secure, and validate drivers against OpenShell's public contract.

Context

This is a remaining deliverable of the OpenShell Drivers roadmap (#1051) and follow-on hardening for the external compute-driver extension point (#1907 / PR #1703).

PR #2181 adds examples/fake-driver-rs, a scriptable out-of-process driver that exercises the public compute_driver.proto service over a Unix socket. The example currently contains implementation and scenario files but no README or published driver-author documentation.

The guide should use that example as executable reference material and connect it to external-driver endpoint testing (#1950) and driver conformance work (#2183 / PR #2182).

Definition of Done

• Add a README for examples/fake-driver-rs covering its purpose, build, launch, configuration, and scripted scenarios.
• Document the public compute-driver RPC and capability/version compatibility expectations.
• Document Unix socket ownership, filesystem permissions, gateway configuration, and the operator-owned process trust boundary.
• Explain lifecycle, watch-stream, callback-endpoint, identity, and unsupported-operation expectations.
• Document how to run external-driver endpoint tests and the conformance suite.
• Add an external-driver authoring page to the published documentation and navigation.
• Cross-link #1051, #1907, #1950, #2181, #2183, and #2182.

Start with examples/fake-driver-rs and its implementation and scenario files, then read the public compute_driver.proto contract. Review the external-driver endpoint tests and conformance suite referenced in #1950 and #2183, along with the published documentation navigation. Done means the example README, authoring page, cross-links, and all listed lifecycle, security, compatibility, testing, and configuration guidance are published.