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;
 }

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

@@ -26,13 +26,6 @@ body {
 .carousel-item {
   height: 32rem;
 }
-.carousel-item > img {
-  position: absolute;
-  top: 0;
-  right: 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;
-}
 .marketing .col-lg-4 p {
   margin-right: .75rem;
   margin-left: .75rem;
@@ -59,12 +49,6 @@ body {
   margin: 5rem 0; /* Space out the Bootstrap <hr> more */
 }
 
-/* Thin out the marketing headings */
-.featurette-heading {
-  font-weight: 300;
-  line-height: 1;
-}
-
 
 /* RESPONSIVE CSS
 -------------------------------------------------- */

site/content/docs/5.1/examples/carousel/index.html

@@ -94,19 +94,19 @@ extra_css:
     <div class="row">
       <div class="col-lg-4">
         {{< placeholder width="140" height="140" background="#777" color="#777" class="rounded-circle" >}}
-        <h2>Heading</h2>
+        <h2 class="fw-normal">Heading</h2>
         <p>Some representative placeholder content for the three columns of text below the carousel. This is the first column.</p>
         <p><a class="btn btn-secondary" href="#">View details &raquo;</a></p>
       </div><!-- /.col-lg-4 -->
       <div class="col-lg-4">
         {{< placeholder width="140" height="140" background="#777" color="#777" class="rounded-circle" >}}
-        <h2>Heading</h2>
+        <h2 class="fw-normal">Heading</h2>
         <p>Another exciting bit of representative placeholder content. This time, we've moved on to the second column.</p>
         <p><a class="btn btn-secondary" href="#">View details &raquo;</a></p>
       </div><!-- /.col-lg-4 -->
       <div class="col-lg-4">
         {{< placeholder width="140" height="140" background="#777" color="#777" class="rounded-circle" >}}
-        <h2>Heading</h2>
+        <h2 class="fw-normal">Heading</h2>
         <p>And lastly this, the third column of representative placeholder content.</p>
         <p><a class="btn btn-secondary" href="#">View details &raquo;</a></p>
       </div><!-- /.col-lg-4 -->
@@ -119,7 +119,7 @@ extra_css:
 
     <div class="row featurette">
       <div class="col-md-7">
-        <h2 class="featurette-heading">First featurette heading. <span class="text-muted">It’ll blow your mind.</span></h2>
+        <h2 class="featurette-heading fw-normal lh-1">First featurette heading. <span class="text-muted">It’ll blow your mind.</span></h2>
         <p class="lead">Some great placeholder content for the first featurette here. Imagine some exciting prose here.</p>
       </div>
       <div class="col-md-5">
@@ -131,7 +131,7 @@ extra_css:
 
     <div class="row featurette">
       <div class="col-md-7 order-md-2">
-        <h2 class="featurette-heading">Oh yeah, it’s that good. <span class="text-muted">See for yourself.</span></h2>
+        <h2 class="featurette-heading fw-normal lh-1">Oh yeah, it’s that good. <span class="text-muted">See for yourself.</span></h2>
         <p class="lead">Another featurette? Of course. More placeholder content here to give you an idea of how this layout would work with some actual real-world content in place.</p>
       </div>
       <div class="col-md-5 order-md-1">
@@ -143,7 +143,7 @@ extra_css:
 
     <div class="row featurette">
       <div class="col-md-7">
-        <h2 class="featurette-heading">And lastly, this one. <span class="text-muted">Checkmate.</span></h2>
+        <h2 class="featurette-heading fw-normal lh-1">And lastly, this one. <span class="text-muted">Checkmate.</span></h2>
         <p class="lead">And yes, this is the last block of representative placeholder content. Again, not really intended to be actually read, simply here to give you a better view of what this would look like with some actual content. Your content.</p>
       </div>
       <div class="col-md-5">