docs: fix Deno setup without package.json

[awnoa]

Description

Add a local node_modules installation step to the Deno setup instructions.

Motivation and Context

Deno projects without a package.json resolve npm packages from the global cache by default. commitlint loads shareable configs through Node.js module resolution, so the documented command fails in that setup. Installing with --node-modules-dir=auto gives commitlint the local module tree it expects.

Fixes #4593.

Usage examples

deno add -D npm:@commitlint/cli npm:@commitlint/config-conventional
deno install --node-modules-dir=auto

How Has This Been Tested?

• Checked the command syntax and option availability with Deno 2.6.3
• Previewed the updated Markdown on GitHub

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have verified that any documentation examples I added/changed actually work.
• I have added tests to cover my changes.
• All new and existing tests passed.
• For a feature/bug fix, my commits follow the test-driven flow.

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[greptile-apps]

Confidence Score: 5/5

This looks safe to merge.

• No blocking issues found in the changed code.

Important Files Changed

Filename	Overview
docs/guides/getting-started.md	Clarifies that the local Node.js module resolution setup command applies to Deno 2.

_{Reviews (2): Last reviewed commit: "docs: clarify Deno 2 requirement" | Re-trigger Greptile}

[escapedcat]

Thanks!