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/dashboard/dashboard.rtl.css

@@ -5,7 +5,6 @@ body {
 .feather {
   width: 16px;
   height: 16px;
-  vertical-align: text-bottom;
 }
 
 /*
@@ -58,7 +57,6 @@ body {
 
 .sidebar-heading {
   font-size: .75rem;
-  text-transform: uppercase;
 }
 
 /*
@@ -68,7 +66,6 @@ body {
 .navbar-brand {
   padding-top: .75rem;
   padding-bottom: .75rem;
-  font-size: 1rem;
   background-color: rgba(0, 0, 0, .25);
   box-shadow: inset 1px 0 0 rgba(0, 0, 0, .25);
 }
@@ -80,8 +77,6 @@ body {
 
 .navbar .form-control {
   padding: .75rem 1rem;
-  border-width: 0;
-  border-radius: 0;
 }
 
 .form-control-dark {