Update table a11y guidance

[rsmithadhoc]

#3505

• Updated a11y considerations for table usage
• Kept link for USWDS' table guidance, but downgraded it to a standard link at the bottom of the list.

[humancompanion-usds]

Please follow the pattern we have in guidance of using the action link to USWDS and then adding our guidance underneath under the header "Additional guidance for VA". You can see an example on this same Table guidance page under Usage.

[rsmithadhoc]

Please follow the pattern we have in guidance of using the action link to USWDS and then adding our guidance underneath under the header "Additional guidance for VA". You can see an example on this same Table guidance page under Usage.

Thanks @humancompanion-usds, updated: