[Doc] Add note about Node 12 deprecation warnings

[jcalcaben]

Description

Adds a note about a deprecation warning that shows up in the log when using Node12. This is caused by a dependency and is noted where we specify Node requirements.

Related Issue

Closes PWA-698

Acceptance

Any developer

Verification Stakeholders

Any developer

Specification

Verification Steps

1. Run HTML preview server

2. Verify note appears on the following pages:

   • /tutorials/pwa-studio-fundamentals/
   • /tutorials/pwa-studio-fundamentals/project-setup/
   • /venia-pwa-concept/features/modular-components/
   • /venia-pwa-concept/setup/

Screenshots / Screen Captures (if appropriate)

Checklist

• I have added tests to cover my changes, if necessary.

• I have updated the documentation accordingly, if necessary.

[PWAStudioBot]

	Fails
🚫	A version label is required. A maintainer must add one.

	Messages
📖	Access a deployed version of this PR here. Make sure to wait for the "pwa-pull-request-deploy" job to complete.
📖	DangerCI Failures related to missing labels/description/linked issues/etc will persist until the next push or next nightly build run (assuming they are fixed).
📖	Associated JIRA tickets: PWA-698.

Generated by 🚫 dangerJS against fc080a8

[devops-pwa-codebuild]

Performance Test Results

The following fails have been reported by WebpageTest. These numbers indicates a possible performance issue with the PR which requires further manual testing to validate.

https://pr-2566.pwa-venia.com : LH Performance Expected 0.85 Actual 0.52, LH Best Practices Expected 1 Actual 0.92, WPT Cache Expected 90 Actual 83.333333333333
https://pr-2566.pwa-venia.com/venia-tops.html : LH Performance Expected 0.75 Actual 0.34, LH Best Practices Expected 1 Actual 0.92
https://pr-2566.pwa-venia.com/valeria-two-layer-tank.html : LH Performance Expected 0.8 Actual 0.41, LH Accessibility Expected 0.9 Actual 0.89, LH Best Practices Expected 1 Actual 0.92, WPT Cache Expected 65 Actual 52.333333333333

[tjwiebell]

Changes look good, but I don't see any deprecation warnings on develop; any chance we solved this, or I'm missing a step to repro?

[sirugh]

I think we should clarify that the warning will say the following:

(node:89176) [DEP0066] DeprecationWarning: OutgoingMessage.prototype._headers is deprecated

[sirugh]

I would be specific about this log, rather than just say "there might be a deprecation warning". That way other deprecation notices, should they arise, are noticed and not ignored.

I should have documented this originally but this is the "expected" warning:

[dpatil-magento]

Verification steps pass, QA Approved.