Mobile sidebar accessibility issues in the docs

On smaller screens when the sidebar is offscreen, there are a few accessibility issues:

* There are no focus styles on the button. For someone tabbing through the page, there is no indication the button has been focused and ready to be interacted with.
* When the sidebar is hidden, it is only done so visually. All of the links and the search input is still being tabbed through, which is pretty confusing for a sighted keyboard user tabbing through things and not having any visual indication of where they are.
* There is no accessible name for the toggle button, so a screenreader user doesn't know what the button is/does.
* When the sidebar is completely hidden and not just visually hidden, there will need to be an `aria-expanded` attribute on the button that will change between `true` and `false` depending on the state of the sidebar.
* There is no label or accessible name for the search input. It needs either a label with visually hidden text if you don't want users to see it, or `aria-label` on the input.

┆Issue is synchronized with this [Jira Task](https://mozilla-hub.atlassian.net/browse/FP-76)