Fix duplicate `.rounded-*` and `.fw-semibold` defs in examples

Description

https://github.com/twbs/bootstrap/commit/c73480c63660d0643434e07c9a522758c21a272d brings new .fw-semibold, .rounded-4 and .rounded-5 into Bootstrap. The refactoring of examples to use more utility classes defined those as well so this PR fixes the duplicates and reajust the values for .rounded-4 to .rounded-6 in examples due to the change of values.

• .fw-semibold was defined in site/content/docs/5.1/examples/sidebars/sidebars.css and set to the same value 600 so can be removed
• .rounded-4 was defined in site/layouts/_default/examples.html and set to .5rem which corresponds to the new value of .rounded-3 so I needed to switch all .rounded-4 in examples to .rounded-3.
• .rounded-5 was defined in site/layouts/_default/examples.html and set to .75rem. There's no such value in the utilities so I dropped the class and set this value directly in the CSS files
  • Note: I slightly changed the value for the "Sign up for free" modal example.
• .rounded-6 was defined in site/layouts/_default/examples.html and set to 1rem which corresponds to the new .rounded-4 utility set as well to 1rem (xl) so I needed to switch all .rounded-6 in examples to .rounded-4.

Motivation & Context

Fix the regressions in the examples

Types 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)
• All new and existing tests passed

Related issues

N/A

Live previews

• Dropdowns example in this PR vs. current dropdowns example
• Features example in this PR vs. current features example
• List groups example in this PR vs. current list groups example
• Modals example in this PR vs. current modals example
• Sidebars example in this PR vs. current sidebars example