Front-end: use documentation link in Experience Manager for components

Per CMS-15692, a documentation link (and possibly a documentation text) will be exposed to the front-end on component level.

Use it to create a clickable  icon, possibly with hover text.

This should be on the same level as the HST-Label for which this exists in community/channel-manager/frontend-ng/src/app/model/entities/component-entity.js:

getLabel() {
  const label = this._meta[HstConstants.LABEL];
  if (label === 'null') {
  return this.getType(); // no label available, fallback to type.
  }

  return label;
}

which gets the label from the HTML comments in the site preview (AFAICS).

So the back end should add an extra attribute there and the front-end can use it.

Above is changed, now using the

/_rp/<component-id>./hippo-default/<locale>

call to expose the documentationLink.