Add ARIA Landmarks to Skeleton HTML

[dainemawer]

Is your enhancement related to a problem? Please describe.
Some a11y validators such as aXe trip up on the fact that ARIA Landmarks can sometimes be missed, or not properly implemented.

Further reading

• https://www.w3.org/TR/wai-aria-practices-1.1/examples/landmarks/main.html
  -https://www.w3.org/TR/WCAG20-TECHS/ARIA11.html

Describe the solution you'd like
We should ensure that fresh installations of the theme-scaffold contain semantically correct, and accessibly valid ARIA Landmarks such as <main aria-labelledby="title">, <header>, <footer> etc in order to pass accessibility validation. Furthermore, the accessible "Skip Links" anchor should be included within a landmark as well as by default within the theme-scaffolds default HTML - we could possibly create a separate ticket for that.

Additional context
This ticket originates from a chat I had with @joesnellpdx regarding an accessibility audit on a new build.