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

build/svgo.yml

@@ -15,6 +15,10 @@ js2svg:
   indent: 2
 
 plugins:
+  # remove this with IE 11 is no longer supported
+  - addAttributesToSVGElement:
+      attributes:
+        - focusable: false
   - cleanupAttrs: true
   - cleanupEnableBackground: true
   - cleanupIDs: true
@@ -41,8 +45,9 @@ plugins:
   - removeHiddenElems: true
   - removeMetadata: true
   - removeNonInheritableGroupAttrs: true
-  - removeTitle: true
-  - removeUnknownsAndDefaults: true
+  - removeTitle: false
+  - removeUnknownsAndDefaults:
+      keepRoleAttr: true
   - removeUnusedNS: true
   - removeUselessDefs: true
   - removeUselessStrokeAndFill: true