fix: EDU BlockRichTable adjustments

[stephiescastle]

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Description

Addresses feedback:

• https://github.com/nasa-jpl/www/issues/171#issuecomment-2370104633

Changes:

• Changes EDU BlockRichTable header row to use bg-jpl-violet-darker
• Modifies min-width rules in BlockRichTable and nested Blocks within to better handle multiple columns

Instructions to test

Check the chromatic Storybook: https://668c47cbeb95392cd79c3c0d-kzqmnpjhzk.chromatic.com/?path=/story/components-blocks-blockrichtable--mixed-column-widths&globals=theme:ThemeEdu

Or test locally:

1. make vue-storybook
2. View http://localhost:6006/?path=/story/components-blocks-blockrichtable--mixed-column-widths&globals=theme:ThemeEdu

Tested in the following environments/browsers:

Operating System

• macOS
• iOS
• iPadOS
• Windows

Browser

• Chrome
• Firefox ESR
• Firefox
• Safari
• Edge

[Scotchester]

Checked it out and it looks good. Not sure if that is a big enough size bump for the images, but let's see what they think.