Docs: Add black text/background examples

[EricDunsworth]

Description

These classes have existed since 5.1 but weren't previously documented.

Specifically:

• .text-black
• .bg-black
• .bg-black.bg-gradient

Motivation & Context

• Most users might not realize these classes exist unless they're covered in the documentation
• Similar examples for the .text-white and .bg-white classes already exist:
  • https://getbootstrap.com/docs/5.2/utilities/colors/#colors
  • https://getbootstrap.com/docs/5.2/utilities/background/#background-color
• Having a .bg-black.bg-gradient example makes sense given that the bg-gradient class has an effect on black backgrounds (unlike .bg-white.bg-gradient)

Type of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Refactoring (non-breaking change)
• Breaking change (fix or feature that would change existing functionality)

Checklist

• I have read the contributing guidelines
• My code follows the code style of the project (using npm run lint)
• My change introduces changes to the documentation
• I have updated the documentation accordingly
• I have added tests to cover my changes
• All new and existing tests passed

Live previews

• https://deploy-preview-37538--twbs-bootstrap.netlify.app/
  • https://deploy-preview-37538--twbs-bootstrap.netlify.app/docs/5.2/utilities/colors/#colors
  • https://deploy-preview-37538--twbs-bootstrap.netlify.app/docs/5.2/utilities/background/#background-color
  • https://deploy-preview-37538--twbs-bootstrap.netlify.app/docs/5.2/utilities/background/#background-gradient

[julien-deramond]

Thanks for your PR @EricDunsworth. Agree that some things are missing in the docs. We're currently working on #35857 so we'll probably look at yours after the dark mode is merged to avoid some conflicts to resolve. We'll get back to you asap :)