LRDOCS-526 PropertiesDocIndexBuilder and targets

@jrao

Hi Hugo,

Executing 'ant propertiesdoc'  builds the properties HTML files to api/propertiesdoc/ and builds an index HTML file for them in api/propertiesdoc/ as well.

The 'propertiesdoc' target is added to the 'doc' target so the properties docs will be generated along with the other docs we upload to docs.liferay.com.

Thanks!
Jim