[doc] Refactor site content navigation

[jcalcaben]

Description

This PR refactors the doc site navigation lists to improve content discovery and separate the different types of docs (i.e. conceptual, tutorials, and reference).

Related Issue

None

Acceptance

@awilcoxa

Verification Stakeholders

@awilcoxa

Verification Steps

1. Navigate to the docs directory: cd pwa-devdocs
2. Remove any existing _site directory: rm -fr _site
3. Build the HTML preview site: npm run develop
4. Verify new top navigation
5. Verify Getting Started and Reference API nav items have a dropdown to more specific items
6. Click through the various items on the top nav
7. Verify items have new left navigation contents and the entry scrolls into view

Screenshots / Screen Captures (if appropriate)

Checklist

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

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