docs: explain dependency injection with Sinon

[Frostand]

Purpose (TL;DR) - mandatory

Fixes #1458 by adding an executable guide that demonstrates plain dependency injection with a Sinon stub, without module interception.

Background (Problem in detail) - optional

The existing how-to material covers link seams, while #1458 asks for a small no-framework dependency-injection example. The guide shows a factory that accepts its API dependency, a production composition root, and a test that injects and asserts a Sinon stub.

Solution - optional

• add the dependency-injection guide and link it from the how-to overview/sidebar
• add an executable documentation test for the returned value and exact call arguments
• update the sidebar generator and generated sidebar together

How to verify - mandatory

1. Check out this branch.
2. Run npm install at the repository root and in docs/.
3. Run the following checks:
   • root npm run build-artifacts
   • root npm run test-node — 1,556 passing; 12 pending
   • root npm run lint
   • root npm run prettier:check
   • docs npm run test:docs — 733 passing
   • docs npm run docs:build

Known baseline limitations: docs-wide lint/format checks report existing issues in untouched tests and generated files, and link checking requires the external lychee binary. The new guide/test and edited generator pass targeted lint/format checks; no dependency or lockfile changed.

Checklist for author

• npm run lint passes
• No uncached standard-library references are introduced by this documentation example.

Tooling disclosure: this change was prepared with OpenAI Codex and validated with the commands above.