docs(codegen): recover Marten DevOps Dockerfile + 'codegen write' guidance (#2846)

[jeremydmiller]

Summary

Closes #2846.

Marten 9.0 removed its runtime code-generation pipeline entirely, and the Marten 9.0 docs cleanup (marten#4498) followed up by stripping the dotnet run -- codegen write Dockerfile section from Marten's DevOps page. That guidance no longer applies to Marten, but Wolverine still ships its own runtime codegen, so the multi-stage build pattern (pre-generate handler/endpoint adapters into Internal/Generated/, ship static C# in the production image) is directly applicable to Wolverine apps.

Replaces the placeholder "Embedding Codegen in Docker" section in docs/guide/codegen.md (which previously pointed at the now-removed Marten page) with the recovered guidance adapted for Wolverine.

What the new section covers

• Wire up the CLI command — note that return await app.RunJasperFxCommands(args); is the last-line requirement to make dotnet run -- codegen write reachable.
• Multi-stage Dockerfile — full SDK → publish → runtime layout, base images bumped to mcr.microsoft.com/dotnet/sdk:9.0-alpine / aspnet:9.0-alpine to match Wolverine 6.0's minimum TFM. Includes the non-root user + EXPOSE 5000 hygiene the Marten original had.
• Caveat the issue called out — "The codegen write step has no runtime resources." Explains why eager DB/broker connections in Program.cs break the build-stage step, with two mitigations: defer to hosted services / DI factories, or use the CodeGeneration.IsRunningGeneration() short-circuit pattern that already lives further down the same page (cross-linked).
• Cross-link to AOT — points at docs/guide/aot.md for the trim/AOT follow-on once a Static-mode image is in place.

Doc home

Picked option #2 from the issue ("new section inside existing docs/guide/codegen.md") because:

1. The existing ## Embedding Codegen in Docker H2 anchor was already cross-referenced from the WolverineFx.RuntimeCompilation section earlier in the page (see [Embedding Codegen in Docker](#embedding-codegen-in-docker) below). Replacing the section in place keeps that link working.
2. No new file means no Vitepress sidebar/nav change.
3. The topic is squarely a codegen-deployment concern, as the issue notes.

Test plan

• git diff clean — pure content swap inside the existing ## Embedding Codegen in Docker section.
• Existing cross-reference from line 182 still resolves to the same anchor (#embedding-codegen-in-docker).
• CI / docs preview

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