docs: document Java packaging on restricted networks

[devin-ai-integration]

Fern's local Java packaging can use corporate proxies and repository mirrors, reuse Docker Gradle caches, and prefer the generated Gradle wrapper on the host. This guidance helps teams build distributable SDKs on restricted networks or when Maven Central rate limits dependency downloads.

Implements docs for: fix(cli): make --package Java builds resilient to Maven Central rate limits and restricted networks (fern-api/fern#17942)

Summary

• Update the SDK commands reference with proxy forwarding, read-only host Gradle configuration, FERN_MAVEN_CENTRAL_MIRROR defaults/overrides, wrapper preference, and errors when no generator qualifies for packaging.
• Link to the network guidance from Java publishing and self-hosted SDK generation.
• Leave generated changelogs unchanged.

Validation

• Verified behavior against the merged source PR diff using git show 617a6caa8d4.
• cd fern && npx --yes fern-api@latest check --warnings: passed, 0 errors and 11 warnings outside the changed content.
• Pre-commit on all three changed files: passed; Vale findings are on unchanged lines.
• git diff --check: passed.

Link to Devin session: https://app.devin.ai/sessions/6483f8d7879743aa8fc0f4ec69b86eb3
Open in Devin Desktop: https://app.devin.ai/desktop/session/6483f8d7879743aa8fc0f4ec69b86eb3?variant=devin

[devin-ai-integration]

I'll fix CI failures and address comments from users with write access. I'll skip comments containing "(aside)".

• Disable automatic comment, CI, and merge conflict monitoring

[github-actions]

🌿 Preview your docs: https://fern-preview-devin-1790960995-java-packaging-networks.docs.buildwithfern.com/learn

Here are the markdown pages you've updated:

• learn/cli-api-reference/cli-reference/sdk-commands
• learn/sdks/deep-dives/self-hosted
• learn/sdks/generators/java/publishing