New userguide index and link

[belbiy]

Description

This pull request updates the documentation search index and site switcher link for user guide.

Related Issue

Acceptance

Verification Stakeholders

Specification

Verification Steps

1. Go to docs page
2. Click on the waffle menu to reveal the site switcher.
3. See that Commerce User Guide now points to http://docs.magento.com/user-guide/

Screenshots / Screen Captures (if appropriate)

Checklist

• I have updated the documentation accordingly, if necessary.
  [ ] I have added tests to cover my changes, if necessary.

[dshevtsov]

Relates to commerce-docs/devdocs-theme#57

[PWAStudioBot]

	Fails
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	Messages
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Generated by 🚫 dangerJS against 0178b30

[jcalcaben]

For some reason, the navigation in the FAQ page does not appear: /frequently-asked-questions/

[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-2418.pwa-venia.com : LH Performance Expected 0.85 Actual 0.57, LH Best Practices Expected 1 Actual 0.92
https://pr-2418.pwa-venia.com/venia-tops.html : LH Performance Expected 0.75 Actual 0.36, LH Best Practices Expected 1 Actual 0.92
https://pr-2418.pwa-venia.com/valeria-two-layer-tank.html : LH Performance Expected 0.8 Actual 0.5, LH Accessibility Expected 0.9 Actual 0.89, LH Best Practices Expected 1 Actual 0.92