open links not generated by jsdoc in new window

For "improve reference documentation page" pull request for the cesium website.
All links have a default of `target="_blank"` so links created by html written in the documentation will open in new pages.  I over-ride this default for links generated by jsdoc to have `target="_self"`.

This is the easiest way I could think of to force external links to open in a new window, which is needed when the reference documentation is embedded.