Provide ability to skip to main content (#949)

Why are these changes being introduced:

* The current tab order requires keyboard or screenreader users to tab
  though all of the navigation links before they get to the main content
  of the page

Relevant ticket(s):

* Discussed as part of larger WCAG ticket https://github.com/just-the-docs/just-the-docs/issues/566

How does this address that need:

* Introduces an off screen link as the first element in the tab order
  that will display when it has focus but remain off screen for users
  not using tab to navigate the document.
* Introduces a new scss file with the required style elements and uses
  colors variables to ensure both light and dark mode are respected.

Background information on this technique:

- https://www.w3.org/TR/WCAG20-TECHS/G1.html
- https://accessibility.oit.ncsu.edu/it-accessibility-at-nc-state/developers/accessibility-handbook/mouse-and-keyboard-events/skip-to-main-content/