Determine and set a main landmark

[adagar]

Is your feature request related to a problem? Please describe
When running Axe assertions, one of the errors is the lack of a main landmark. Landmarks are an essential tool for people using non-traditional modes of navigation, so this is a change that has high-impact for a11y-users and little to no impact for other users.

Describe the solution you'd like
An element, likely the DOM node that contains the visual and code view of elements should be changed from a generic div element to a main one.

NOTE: Due to the nature of nested documents (https://www.w3.org/TR/wai-aria-practices/examples/landmarks/main.html#:~:text=The%20main%20landmark%20should%20be,should%20have%20a%20unique%20label.) a <main> landmark should also be determined for the content inside of the iFrame, though I understand that this content is considered user generated and beyond the scope of core story-book features.

Are you able to assist to bring the feature to reality?
Yes, I can. This should be a small change.

Additional context
Add any other context or screenshots about the feature request here.

[shilman]

Olé!! I just released https://github.com/storybookjs/storybook/releases/tag/v6.2.0-alpha.26 containing PR #13827 that references this issue. Upgrade today to the @next NPM tag to try it out!

npx sb upgrade --prerelease

Closing this issue. Please re-open if you think there's still more to do.