Docs: replace CSS by utilities in examples (#35699)

* Drop .opacity-50 and .opacity-75 redefinition in examples

* Drop unused .card-img-right from blog example CSS files

* Use line-height utilities when possible

* Use rounded-* utilities in examples

* Replace .nav-underline by .nav-scroller and use it in examples.html default

* Use .mb-1 for .blog-post-title

* Remove unused CSS rule and use .fw-* utilities for carousels examples

* Use utilities for cheatsheet examples

* Extract some CSS to utilities for .nav-masthead .nav-link in cover example

* Dashboard group of minor modifications

* Dropdowns example: refactoring

* Dropdowns example refactoring: fix linting by removing selector by id

* Features example refactoring

* Headers example refactoring

* List groups example refactoring

* Sidebars example refactoring

* Sign-in example refactoring

* Starter template refactoring

* Fix RTL examples

Co-authored-by: Mark Otto <markd.otto@gmail.com>

site/content/docs/5.1/examples/carousel/carousel.css

@@ -26,13 +26,6 @@ body {
 .carousel-item {
   height: 32rem;
 }
-.carousel-item > img {
-  position: absolute;
-  top: 0;
-  left: 0;
-  min-width: 100%;
-  height: 32rem;
-}
 
 
 /* MARKETING CONTENT
@@ -43,9 +36,6 @@ body {
   margin-bottom: 1.5rem;
   text-align: center;
 }
-.marketing h2 {
-  font-weight: 400;
-}
 /* rtl:begin:ignore */
 .marketing .col-lg-4 p {
   margin-right: .75rem;
@@ -63,8 +53,6 @@ body {
 
 /* Thin out the marketing headings */
 .featurette-heading {
-  font-weight: 300;
-  line-height: 1;
   /* rtl:remove */
   letter-spacing: -.05rem;
 }