docs: add "Writing an integration" guide

[lesnik512]

What

Adds a published docs page, Integrations → Writing an integration, aimed at agents authoring official modern-di integrations.

The guide is distilled from the five existing integration packages (modern-di-fastapi, -faststream, -litestar, -typer, -pytest) and covers:

• The integration contract — the five things every integration provides (connection ContextProviders, setup_di, fetch_di_container, per-unit-of-work child-container builder, FromDI + Dependency resolver), each with signature and responsibility.
• Lifecycle rules — reopen-root-on-startup to survive restarts (ContainerClosedError), always close the child in finally, close_async/close_sync matching.
• Scope mapping table (request → REQUEST, websocket → SESSION, message/command → REQUEST, nested action → ACTION).
• Realization table — how each of the four app integrations implements each contract point, plus a pytest note for test-runner integrations.
• Repo scaffolding — naming, main.py + re-exporting __init__.py, pyproject.toml shape, test layout, mirrored tooling, and a docs/nav entry in this repo.
• Planning — recommends following planning-convention.
• A copy-pasteable checklist.

Adds the page to the mkdocs.yml Integrations nav.

Verification

just docs-build (mkdocs build --strict) passes — broken links or nav warnings would fail the strict build.

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