Adds two positioning pages under Introduction (the docs had none).

introduction/comparison.md — modern-di vs other libraries

• Opens with "Do you even need a DI container?" — honestly points single-service FastAPI/Litestar users at native Depends/Provide, and names the break-even triggers (second entrypoint; typed app-scoped singletons; off-request resolution; whole-app test overrides).
• A landscape table (modern-di / Dishka / dependency-injector / injector / Depends).
• Honest per-competitor prose: Dishka is the closest library and an excellent choice for custom scopes / async / breadth; modern-di's real edges are the first-party pytest plugin, sync-only simplicity, and the all-official MIT stack. dependency-injector noted as actively maintained again after an earlier hiatus.

introduction/that-depends-or-modern-di.md — the chooser

• New project → modern-di; existing that-depends → maintained and production-proven, migrate only for scopes / no-global-state. Diff table + "choose X if…", linking the existing migration guide.

Verified: mkdocs build --strict passes; both pages added to the Introduction nav; no link warnings.

Part of the modern-python promotion strategy (DI positioning backbone).