Add ARIA search landmark to documentation.

[bardiharborow]

Reference: WAI-ARIA Authoring Practices 1.1 § 4.3.8.

Addendum: .bd-sidebar and bd-toc don't appear to have enough roles, but I'm new at this whole ARIA thing so would prefer advice on this one.

[patrickhlauke]

i'd say .bd-toc is fine as is. .bd-sidebar probably can be seen as a navigation, but so does the stuff in the header...when you then have more than two containers acting as navigation, they need to be disambiguated with some form of label/name (e.g. an aria-label) so it's clear what the difference is (like "general site navigation" and "documentation navigation" or similar). draft some PR up and we can workshop it :)