PR: Updated "Adding icons to buttons" section

[bbrennanbasis]

This PR closes #221. Updated this section of the Button article to include the mention of the Icon component.

Note:

There are several other PRs that should be approved before this one is merged:

• PR #283
• PR #282
• From webforj-doc-samples: PR #89 Removal of html tags and dwc-icons.

[hyyan]

@MatthewHawkins give this a quick scan and merge please