Docs: fix text rendering of navbar external content

[julien-deramond]

Description

Reference: #38430 (comment)

Before this PR, in https://twbs-bootstrap.netlify.app/docs/5.3/components/navbar/#external-content, when you click on the toggle button, "Toggleable via the navbar brand." is not visible because written with the same color as the background.

This is due to the fact that .text-muted is deprecated and has been changed by .text-body-emphasis.
In light mode, .text-body-emphasis will have the same color as .bg-dark.
Based on what's explained in https://twbs-bootstrap.netlify.app/docs/5.3/components/navbar/#color-schemes, we can simply use data-bs-theme="dark" combined with the .bg-dark to keep using .text-body-emphasis. But we should also rather use .text-body-emphasis for the white color.

Another possibility to fix this issue would be to avoid using a dark navbar here since the content is not linked to this specific color, we could have a light background to explain this feature.

Type of changes

• Bug fix (non-breaking change which fixes an issue)

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-38431--twbs-bootstrap.netlify.app/docs/5.3/components/navbar/#external-content

Related issues

#38430