Draft guidance section on input assistance and validation states and properties

[mcking65]

This section will cover:

• [ ] aria-required
• [ ] aria-invalid
• [ ] aria-placeholder
• [ ] aria-describedby
• [ ] aria-errormessage
• [ ] Setting focus after submit with errors (suggested in issue #624)
• [ ] Use of aria-live to assist users (suggested by issue #624)

Preview links for work in progress

Work on this issue is in the issue252-input-assistance branch. Current drafts:

• Forms pattern
• Forms example

[carmacleod]

I think this guidance is valuable, and we should put in the effort required to finish it.

It would also be good to add a Note about aria-required on checkbox (as explained in https://github.com/w3c/aria/issues/921#issuecomment-708432172).

In addition, consider creating an example of a group with aria-required.