Site update

Summary:
- Inject best practices from H5BP.
- Adopt Bootstrap front-end framework.
- Support hosting of multiple document versions with a 'document-switcher'. Closes #1.
- Fix most of the responsive web design issue. Closes #2.
- Miscellaneous front-end updates, like:
  * Navigation pills with dropdown that are auto collapsed to a button on extra small viewport.
  * Change to wider container for easier documentation viewing.
  * Category-aware pager for news post entries.
- Correct some of the semantic issues from the Jekyll side. Semantic issues from Doxygen side as reported in #3 can only be solved by making changes in the Urho3D main repo during documentation building time.