Update DndContext Props definition, references to "accessibility" prop

[sanrafa]

Apologies for the Prettier churn, this is a smaller change than it appears. Similarly to #14, I noticed that the Accessibility guide doesn't reflect the DndContext prop type update.

This PR makes the following changes:

• Update the Props definition in api-documentation/context-provider to reflect the current version.
• Change DndContext prop references in guides/accessibility to accessibility.[propName] so it's clearer they belong in an object under the accessibility prop.
• Add an example to the screen reader instructions section of guides/accessibility so users have a clearer idea of how to customize instructions.

This is my first ever contribution, so I'm more than happy to accept feedback. :)

Many thanks to the maintainers of this library! It's been a godsend for my project, and I really appreciate how a11y was taken into account. Hopefully these changes can help make it easier for newer devs like me to implement 👍