Fixed the namespace and class icon issue and updated the Doxygen file.

While generating the 2.2 documentation for the website, I noticed that the Doxygen and the CSS files were outdated, thus I've made some adjustments.

While doing so I've now also link the JavaScript files in the header, thus the JavaScript powered bits in the documentation now work.

Additionally I changed the file ending to `.html` instead of `.htm` and made everything use UTF-8.
Fixed :wink: