Add note on netlify config when migrating Node.js

[tlylt]

What is the purpose of this pull request?

• [x] Documentation update
• [ ] Bug fix
• [ ] Feature addition or enhancement
• [ ] Code maintenance
• [ ] DevOps
• [ ] Improve developer experience
• [ ] Others, please explain:

Overview of changes: Fixes #2246

Anything you'd like to highlight/discuss: nil

Testing instructions: https://deploy-preview-2554--markbind-master.netlify.app/devguide/development/migratingnodejs

Proposed commit message: (wrap lines at 72 characters) Add note on netlify config when migrating Node.js

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Checklist: :ballot_box_with_check:

• [ ] Updated the documentation for feature additions and enhancements
• [ ] Added tests for bug fixes or features
• [x] Linked all related issues
• [x] No unrelated changes

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Reviewer checklist:

Indicate the SEMVER impact of the PR:

• [ ] Major (when you make incompatible API changes)
• [ ] Minor (when you add functionality in a backward compatible manner)
• [x] Patch (when you make backward compatible bug fixes)

At the end of the review, please label the PR with the appropriate label: r.Major, r.Minor, r.Patch.

Breaking change release note preparation (if applicable):

• To be included in the release note for any feature that is made obsolete/breaking

Give a brief explanation note about:

• what was the old feature that was made obsolete
• any replacement feature (if any), and
• how the author should modify his website to migrate from the old feature to the replacement feature (if possible).

[github-actions[bot]]

@tlylt Each PR must have a SEMVER impact label, please remember to label the PR properly.