Docs: change colors of position marker example to work with dark mode

[julien-deramond]

Description

This PR suggests changing the color of the marker shown in our docs in "Position > Examples" so that the colors can work either in light or dark modes.

The black color was better in light mode but we don't have any utilities to do the opposite (white color) in dark mode.

Motivation & Context

When https://twbs-bootstrap.netlify.app/docs/5.3/utilities/position/#examples is displayed in dark mode, one can't see the marker's shape:

Type of changes

• Enhancement (non-breaking change which adds 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
• (N/A) I have added tests to cover my changes
• All new and existing tests passed

Live previews

• https://deploy-preview-38445--twbs-bootstrap.netlify.app/docs/5.3/utilities/position/#examples

Related issues

Closes #38444

[julien-deramond]

Linked to this PR, when I do this kind of change, it confirms somehow that we'd need some utilities that switch colors in light/dark mode. We have .bg-dark and .bg-black that stay black. We need something that is black in light mode and white in dark mode and could be named .bg-opposite or something else.