1.0 Docs Updates

[dfreeman]

This PR contains a potpourri of small updates to the documentation site in preparation for 1.0:

• Restructuring the Contents sidebar to better account for some of the new top-level pages we've added
• Adding one last 0.9 -> 1.0 migration note around the type of {{component 'foo'}}
• Clarifying a few sections and adding some tips in response to some long-standing open question issues
• Removing the "we're pre-1.0 and unstable" warnings from various pages (🎉)

Obviously we can continue to iterate and expand after the stable release, but I believe this covers the last odds and ends that I knew of wanting to get straightened up before then.

[chriskrycho]

Other than that one typo, this looks great!