Various HTML & design changes

* Fixed a few HTML validation errors
* Removed prettify loading on the pages where it wasn't being used
* Removed request for non-existent `bootstrap-responsive.min.css` file: this file is no longer needed with bootstrap 3+ [Ref](http://bootply.com/bootstrap-3-migration-guide)
* Prettified the welcome banner
* Prettified the download blocks
* Prettified the documentation blocks