[docs] Add focus indicator style guidance

[hellogreg]

Because we currently have so many disparate keyboard focus styles across our site system (dashed/dotted/solid/inset outlines, background highlights, text color changes, etc.), we're standardizing these indicators to align with WCAG 2.2's recommendation:

Solid, offset outlines of at least 2px with at least 3:1 background/adjacent contrast

We need to craft some verbiage, add some relevant imagery, find a logical home for this at ux.redhat.com, and get the word out to our designers and engineers.

We've already got some documentation on this topic strewn across the site (e.g. the CTA Styles page), some of which we can consolidate for this issue.

References:

• Focus styles at RHDC discovery deck
• CodePen of proposed styles
• RHDS CTA focus guidance
• WCAG 2.2 SC 2.4.13: Focus Appearance