[www] IA redesign: Allow top-level links to pages

[fk]

The current implementation for the "Docs", "Tutorial" and "Features" sidebars only allows for links to pages to exist within the items object of a top-level item:

gatsby/www/src/data/sidebars/doc-links.yaml

Lines 1 to 12 in 7ed8da9

	- title: Quick Start
	items:
	- title: Getting Started
	link: /
	- title: Starters
	link: /gatsby-starters/
	- title: Deploying
	link: /deploy-gatsby/
	- title: Community
	link: /community/
	- title: Awesome Gatsby
	link: /awesome-gatsby/

Top-level items currently only are used to "group" items in an accordion and are not expected to link to a page.

The doc IA RFC however forsees top-level items to be able to link to pages, too—see "Introduction", "Quick Start" and "Commands (Gatsby CLI)" here:

This issue captures the need to extend the implementation to allow top-level links to pages.