Adding clarification for responsive pattern of fixed-width columns

[ajbush]

Adding one sentence under "Fixed-width columns" on the Tables page to clarify the responsive pattern for breakpoints 600 and less. Chuck and our team have been working on implementing this for use in the CMS and wanted to document the responsive pattern.

Review

• @marteki
• @mistergone
• @schaferjh
• @jimmynotjim
• @Scotchester
• @caheberer

Preview this PR without the whitespace changes

Checklist

• [ ] Changes are limited to a single goal (no scope creep)
• [ ] Code can be automatically merged (no conflicts)
• [ ] Code follows the standards laid out in the front end playbook
• [ ] Passes all existing automated tests
• [ ] New functions include new tests
• [ ] New functions are documented (with a description, list of inputs, and expected output)
• [ ] Placeholder code is flagged
• [ ] Visually tested in supported browsers and devices
• [ ] Project documentation has been updated (including the "Unreleased" section of the CHANGELOG)

[caheberer]

👍