docs: document Go source zip artifact for fern generate --pack

[devin-ai-integration]

Summary

fern generate --pack now produces an artifact for Go SDKs: a zip of the generated module source. Previously Go was the one language where --pack produced nothing (Go modules have no binary package format), so teams sharing a generated Go SDK internally had no shareable file. This documents the new fern-dist/<output-dir>-source.zip artifact and how a consumer uses it via a replace directive in go.mod.

Implements docs for: feat(cli): --pack builds a source zip artifact for Go SDKs (fern-api/fern#17293)

Stacks on top of #6468, which documents --pack / --pack-mode generally; this PR adds only the Go-specific content. Base it on main once #6468 merges.

Changes in fern/products/cli-api-reference/pages/commands.mdx, inside the ### pack section:

• Artifact table row for Go now names the file: module source zip (<output-dir>-source.zip).
• One paragraph plus a go.mod snippet covering: the zip excludes fern-dist/ and .git/, it's built in-process (same artifact in host and docker pack modes, no Go toolchain required), and consumers unzip it and add a replace directive.

Link to Devin session: https://app.devin.ai/sessions/d2a296bf5ee54923be223cf096a1a852

[devin-ai-integration]

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