Target adds Source URL to articles

Hi Rich,

I added the capability to add each article's GitHub source URL to the article's header. In the Ant buildfile, I renamed target *number-headers* to *add-article-metadata* and added the call to this new Java class com.liferay.documentation.util.AddSourceUrlSiteMain.

https://issues.liferay.com/browse/LRDOCS-1392

Related Jira tasks
- https://issues.liferay.com/browse/LPS-49798 KBArticles now have a source URL field. If an article has a source URL value, the display portlet shows an icon with a reference to the GitHub source URL. (Adolfo)
- https://issues.liferay.com/browse/LPS-49852 KB handles a Markdown file's sourceUrl value adding it to the KB Article. (Jim) (Adolfo is reviewing)