Improves accessibility on tables by adding captions documentation and scope="col" on thead th

Created by: andresgalante

This PR is related to #23746 (closed) but it doesn't solve it.

This PR introduces scope="col" on tables thead ths following WCAG recommendations: https://www.w3.org/WAI/tutorials/tables/

It also introduces a section to show how caption is displayed and when/how to use them. https://www.w3.org/WAI/tutorials/tables/caption-summary/

What do you think?