docs: document fern generate --local in GitHub Actions

[devin-ai-integration]

Summary

Adds a Run in GitHub Actions section to the Self-hosted SDKs page (fern/products/sdks/deep-dives/self-hosted.mdx) to answer whether fern generate --local is supported in CI and what the runner needs.

The page already documented the infrastructure requirements (Docker daemon / DinD, FERN_TOKEN, outbound access, output write access), private-registry mirroring, and the self-managed versioning caveat, so this is a targeted addition rather than a new page:

• States --local works in GitHub Actions and that ubuntu-latest ships Docker preinstalled (self-hosted runners need DinD or a mounted socket), linking back to the existing #infrastructure-requirements section.
• Adds a minimal workflow example: checkout, setup-node, npm install -g fern-api, fern generate --group python-sdk --local with FERN_TOKEN from a repository secret.
• Notes the two gotchas: generator images aren't cached between jobs (first generation is slower), and versions are computed by your pipeline, linking to the existing Self-hosted SDK versioning guidance.

No changelog entry (SDK feature). Vale passed; fern check --warnings reports 0 errors.

Link to Devin session: https://app.devin.ai/sessions/5ccfdd84f65a4ce7b5b4587558414201

[devin-ai-integration]

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[github-actions]

🌿 Preview your docs: https://fern-preview-devin-1785769764-local-generate-github-actions.docs.buildwithfern.com/learn

Here are the markdown pages you've updated:

• learn/sdks/deep-dives/self-hosted