docs: add brownfield integration guide for existing .NET apps

[antosubash]

Summary

Adds a new "Brownfield Integration" page under Getting Started that walks teams with large, existing .NET applications through adopting SimpleModule incrementally.

The guide is structured around four migration phases (Coexist → First module → Cross-cutting modules → Module-first), so readers can stop at any stable end-state rather than committing to a full rewrite.

What's covered

• Phase 1 (Coexist): exact csproj and Program.cs changes to stand the framework up next to legacy controllers, MVC, EF Core, and existing auth — with no behavioural change.
• Phase 2 (First module): how to pick a low-risk candidate, scaffold via sm new module, and bridge to legacy data via an adapter before moving the schema.
• Phase 3 (Cross-cutting modules): a catalogue of what each cross-cutting module replaces, when to adopt it, and a parallel-run pattern for safe cutovers.
• Phase 4 (Module-first): what a healthy steady-state looks like.
• Brownfield-specific notes on authentication (no OpenIddict requirement, fallback policy gotcha), database coexistence (shared connection + per-module schemas, or per-module connection strings), and frontend integration (skipping Inertia/React entirely for API-only hosts).
• A "common pitfalls" section for the issues most likely to bite during integration.

The guide is grounded in the actual public API surface — builder.AddSimpleModule() / await app.UseSimpleModule(), generator analyzer reference shape, Database config section, fallback RequireAuthenticatedUser policy — so the snippets are copy-pasteable.

Files changed

• docs/site/getting-started/brownfield-integration.md — new page (~360 lines).
• docs/site/.vitepress/config.ts — sidebar entry added under Getting Started.

Test plan

• npm run docs:dev (or local VitePress preview) — new page renders, sidebar entry appears under Getting Started.
• Internal links resolve: /getting-started/quick-start, /guide/modules, /guide/endpoints, /guide/database, /advanced/source-generator.
• Code snippets compile mentally against current Program.cs shape in template/SimpleModule.Host.

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