SVG accessibility fixes (#27826)

* SVG accessibility fixes

- add `focusable="false"` to all SVGs, to stop IE from including the `<svg>` in its default focus cycle
- completely hide decorative SVGs using `aria-hidden="true"`
- add `role="img"` to meaningful/"content" SVGs, ensure they have an appropriate `<title>` and/or `aria-label`
- simplify placeholder default title

* Simplify usage of placeholder.svg in the docs

- in most cases, no need for separate/redundant text and title (now that we make text itself part of the alternative text explicitly)
- no need (expect in rare cirumstances) to describe the placeholder image (unless the look of the image is important/the actual content, as is the case with the image thumbnail class)

* Simplify example.html rewriting of <svg> to <img> code sample

- as the `alt` for the placeholder isn't really important, just add an ellipsis instead (otherwise, to be correct, we'd have to construct the whole "title+text" construct like we do now in the
placeholder.svg itself

* Change svgo configuration to retain role attribute and add/force focusable=false

- regarding `focusable=false`, see https://github.com/svg/svgo/issues/1011 (which would be the "proper" SVGO fix) and the (hacky) solution https://github.com/svg/svgo/pull/817

site/docs/4.1/content/figures.md

@@ -11,7 +11,7 @@ Use the included `.figure` , `.figure-img` and `.figure-caption` classes to prov
 
 {% capture example %}
 <figure class="figure">
-  {% include icons/placeholder.svg width="400" height="300" class="figure-img img-fluid rounded" title="A generic square placeholder image with rounded corners in a figure." %}
+  {% include icons/placeholder.svg width="400" height="300" class="figure-img img-fluid rounded" %}
   <figcaption class="figure-caption">A caption for the above image.</figcaption>
 </figure>
 {% endcapture %}
@@ -21,7 +21,7 @@ Aligning the figure's caption is easy with our [text utilities]({{ site.baseurl
 
 {% capture example %}
 <figure class="figure">
-  {% include icons/placeholder.svg width="400" height="300" class="figure-img img-fluid rounded" title="A generic square placeholder image with rounded corners in a figure." %}
+  {% include icons/placeholder.svg width="400" height="300" class="figure-img img-fluid rounded" %}
   <figcaption class="figure-caption text-right">A caption for the above image.</figcaption>
 </figure>
 {% endcapture %}

site/docs/4.1/content/images.md

@@ -11,7 +11,7 @@ toc: true
 Images in Bootstrap are made responsive with `.img-fluid`. `max-width: 100%;` and `height: auto;` are applied to the image so that it scales with the parent element.
 
 <div class="bd-example">
-  {% include icons/placeholder.svg width="100%" height="250" class="bd-placeholder-img-lg img-fluid" text="Responsive image" title="Generic responsive image" %}
+  {% include icons/placeholder.svg width="100%" height="250" class="bd-placeholder-img-lg img-fluid" text="Responsive image" %}
 </div>
 
 {% highlight html %}
@@ -42,8 +42,8 @@ In addition to our [border-radius utilities]({{ site.baseurl }}/docs/{{ site.doc
 Align images with the [helper float classes]({{ site.baseurl }}/docs/{{ site.docs_version }}/utilities/float/) or [text alignment classes]({{ site.baseurl }}/docs/{{ site.docs_version }}/utilities/text/#text-alignment). `block`-level images can be centered using [the `.mx-auto` margin utility class]({{ site.baseurl }}/docs/{{ site.docs_version }}/utilities/spacing/#horizontal-centering).
 
 <div class="bd-example bd-example-images">
-  {% include icons/placeholder.svg width="200" height="200" class="rounded float-left" title="A generic square placeholder image with rounded corners" %}
-  {% include icons/placeholder.svg width="200" height="200" class="rounded float-right" title="A generic square placeholder image with rounded corners" %}
+  {% include icons/placeholder.svg width="200" height="200" class="rounded float-left" %}
+  {% include icons/placeholder.svg width="200" height="200" class="rounded float-right" %}
 </div>
 
 {% highlight html %}
@@ -52,7 +52,7 @@ Align images with the [helper float classes]({{ site.baseurl }}/docs/{{ site.doc
 {% endhighlight %}
 
 <div class="bd-example bd-example-images">
-  {% include icons/placeholder.svg width="200" height="200" class="rounded mx-auto d-block" title="A generic square placeholder image with rounded corners" %}
+  {% include icons/placeholder.svg width="200" height="200" class="rounded mx-auto d-block" %}
 </div>
 
 {% highlight html %}
@@ -61,7 +61,7 @@ Align images with the [helper float classes]({{ site.baseurl }}/docs/{{ site.doc
 
 <div class="bd-example bd-example-images">
   <div class="text-center">
-    {% include icons/placeholder.svg width="200" height="200" class="rounded" title="A generic square placeholder image with rounded corners" %}
+    {% include icons/placeholder.svg width="200" height="200" class="rounded" %}
   </div>
 </div>