build(aio): auto-fill width/height to all image tags

Parse all `<img>` tags, during doc-gen, and insert the width and height of
the sourceed image, if neither are already specified.

Warnings are reported if the `<img>` tag has no `src` attribute or the image
cannot be loaded.

The work is done in the `addImageDimensions` post-processor, which must be
configured with a `basePath` so that it knows where to find the images.

Closes #15888

aio/tools/transforms/angular-base-package/processors/copyContentAssets.js

@@ -1,6 +1,7 @@
 
 module.exports = function copyContentAssetsProcessor(copyFolder) {
   return {
+    $runBefore: ['postProcessHtml'],
     assetMappings: [],
     $process() {
       this.assetMappings.forEach(map => {