Cleaning up description of GNU GPL in the index.html and changing the note in the gplv2 and gplv3 license pages

I made some minor changes to how the GNU GPL is described in the index.html page as well a change to the note on the GPLv2 and GPLv3 license pages stating that all source code files should state their copyright and licensing terms, which I think could make sense for all other licensing pages as well.