Clean up and improve cgcore.html

[alanorth]

This cleans up the HTML and CSS to be more HTML5 compliant and use Bootstrap helper CSS classes instead of hardcoding margins, etc, which fixes a few minor glitches in the page. I also made minor improvements to the style based on the Dublin Core website layout and added a visual indicator for the left navbar. See:

I have not changed any of the language, grammar, or schema in pull request. There are still some things we can do to make the CG Core schema reference more readable and helpful after this is merged.

[ayabowork]

Thanks so much, this looks nice @alanorth