Docs: replace CSS by utilities in examples

Description

• 89050651: Drop .opacity-50 and .opacity-75 definition since those utilities already exist
• 146c5963: Remove unused .card-img-right.
• b3421e15: Use .lh-1 and .lh-sm when possible.
• 1ece9c4c: Move .rounded-4, .rounded-5 and .rounded-6 rules directly within site/layouts/_default/examples.html so that several examples can use those extended rounded utilities. Remove border-radius definitions when possible in CSS files and replace them by .rounded-*.
• 4979bcb3: Replace .nav-underline by .nav-scroller in order to share common definitions of .nav-scroller and .nav-scroller .nav in the default example HTML file.
• 9ee29fef: Extract the .mb-1 rule of .blog-post-title.
• 069f102f: Remove unused .carousel-item > img rule for carousels examples (there is no more <img>). Use .fw-* utilities when possible.
• 106067d9: Use several utilities for cheatsheet examples.
• d0d94c03: Use some utilities for .nav-masthead .nav-link in cover example.
• 6ef8f96e: Dashboard example refactoring
• 90577d46: Multiple minor modifications for the Dropdown example
• d2e36849: Fix linting issues due to the previous commit so I chose to drop the ids and create .w-000px classes instead in Dropdown example
• 24f02854: Features example refactoring
• 8d3332eb: Headers example refactoring
• b574af42: List groups example refactoring
• 202af505: Sidebars example refactoring
• 9b8744a9: Sign-in example refactoring
• ca4dba18: Starter template refactoring
• 6cde9998: And finally some fixes mainly for RTL examples because I've apparently forgotten them at the beginning 😇

Checklist

• blog.css / blog.rtl.css
• carousel.css / carousel.rtl.css
• cheatsheet.css / cheatsheet.rtl.css
• form-validation.css
• cover.css
• dashboard.css / dashboard.rtl.css
• dropdowns.css
• features.css
• grid.css
• headers.css
• heroes.css
• list-groups.css
• modals.css
• navbar-top-fixed.css
• navbar-top.css
• navbar.css
• offcanvas.css
• pricing.css
• product.css
• sidebars.css
• signin.css
• starter-template.css
• sticky-footer.css
• sticky-footer-navbar.css

Motivation & Context

Following this comment, this change will reduce the amount of specific CSS used by Bootstrap examples.

Types of changes

• Refactoring (non-breaking change)

Checklist

• I have read the contributing guidelines
• My code follows the code style of the project (using npm run lint)

Related issues

Live previews

For non-regression testing:

• Blog example
• Blog RTL example
• Carousel example
• Carousel RTL example
• Cheatsheet example
• Cheatsheet RTL example
• Cover example
• Dashboard example
• Dashboard RTL example
• Dropdowns example
• Features example
• Headers example
• List groups example
• Modals example
• Offcanvas Navbar example
• Sidebars example
• Sign-in example
• Starter template