Adding Vercel Toolbar to docs. (#6836)
### Description
To create a better feedback loop with the community, we can enable
comments on our documentation.
This PR enables the Vercel Toolbar for local branches and production. On
previews, the toolbar will always be present.
In a typical product, you wouldn't display your toolbar for any site
visitor to use on production. However, I'm doing an overhaul of our
documentation so I'm interested in collecting as much feedback as
possible. Depending on the quantity and quality of comments we get, we
may or may not choose to keep Toolbar active on production long term.
### Testing Instructions
Click the new button in the top right of the preview to enable the
Vercel Toolbar. Leave a comment and resolve it! Fun!
Note that this may be awkward to test on a preview deployment since
previews always have the toolbar. You will likely want to take a look at
the behavior locally by pulling the branch and using `cd docs && pnpm
build && pnpm start`.
