Fix link text and hover color contrast for all bgs

[chalin]

• Closes Link hover color contrast is insufficient when block bg is dark #1482
• Closes Link text and hover color contrast in footer is insufficient #1483
• As was mentioned in the two previous issues, these problems existed before the migration to BSv5. I would have normally deferred resolution of non-BSv5 problems until after the BSv5 release of Docsy, but I had the work done already 🤷🏼‍♂️
• Contributes to Cleanup: don't hard-code styles into layouts #783 for the Docsy footer (partial work)
• Switches from .bg-dark to Docsy's custom style .td-box--dark for the footer, since that style has link and hover colors appropriately set in the latter
• For each .td-box--*, the hover color is determined in a way that is similar to what Bootstrap does, in particular, using $link-shade-percentage

Preview: https://deploy-preview-1525--docsydocs.netlify.app/about

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Screenshots

These images show the fix in a block and the footer. The link contribution guidelines (roughly in the middle of the page) is in a hover state.

Before:

After:

[geriom]

LGTM, Thank you!