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/_includes/example.html

@@ -34,16 +34,10 @@
         {%- assign image_class = image_class[1] | replace: 'bd-placeholder-img-lg', '' | strip -%}
       {%- endif -%}
 
-      {%- assign image_alt = include.content
-        | replace: '<title>', '<title>✂️'
-        | replace: '</title>', '✂️</title>'
-        | split: '✂️' -%}
-      {%- assign image_alt = image_alt[1] -%}
-
       {%- for content_chunk in modified_content -%}
         {%- if content_chunk contains '<svg class="bd-placeholder-img' -%}
           {%- capture img_placeholder -%}
-            <img src="..." {% if image_class %}class="{{ image_class }}" {% endif %}alt="{{ image_alt }}">
+            <img src="..." {% if image_class %}class="{{ image_class }}" {% endif %}alt="...">
           {%- endcapture -%}
           {{- img_placeholder -}}
         {%- else -%}