Add <caption> elements to all documentation tables

**Describe the bug**
Documentation tables on the baseline site don't have caption elements. It isn't required, but it would be nice to get them in there.

**To Reproduce**
Steps to reproduce the behavior:
1. Go to https://baseline.10up.com/component/accordion/
2. Scroll down to a table
4. See error

**Expected behavior**
Captions should exist on table elements throughout the site.