Add keyboard interaction design document to site

[jessiehuff]

We'd like to add our keyboard interaction design document (which outlines the expected keyboard interaction patterns) to the site under Design Documentation > Usage and Behavior.

[dlabaj]

We should look into adding keyboard a11y doc to current framework or keep it in mind when we are making changes to our documentation framework.

[nicolethoen]

I'm in favor of building out a singular page in the design docs about keyboard interaction patterns. Some proposals before have included documenting keybaord interactions per component. I'm not as enthusiastic about the level of effort that'd require.

some extra docs which have been created around keyboard a11y patterns are:

• https://docs.google.com/document/d/1PV7jETvb0-fT985MzIKbRETIcl57ZXeUNXVIPzZMPJ8/edit#heading=h.ebz26dvbqb7s
• https://docs.google.com/document/d/1DWVdV6xjrtea2Ie5T-NUU-tDII6xsJ7evq-Y7FO8ygU/edit