Accessibility: Add tips for fixing common screen reader problems

Created by: courier-new

Some initial high-value takeaways for improving screen reader navigation, collected from the first round of accessibility audits our team completed and based around practices we're already following in our codebase. Notably, suggesting <VisuallyHidden /> (as opposed to Bootstrap .sr-only) for providing "alt text" that's only exposed to accessibility tools for icon buttons and other information primarily communicated graphically (thanks @LawnGnome for surfacing this in your a11y PR, since it was basically tribal knowledge).

Test plan

Just a docs change.