Misc. enhancements and fixes.

- Add support for hosting multiple document versions.
- Remove web 'author' meta tag to avoid confusing it with main source code and content author of Urho3D. Instead, just put the credit in the humans.txt.
- Fix site generation performance regresssion issue due to usage of pages_list in the default.html to list the site navigation dropdowns. Revert back to list them explicitly using header_navs.yml data file.
- Remove temporary fix to match Doxygen's heading styles to Jekyll's heading styles. The semantics issue will be addressed separately and is now being tracked in issue #3.
- Enhance the pages.html to only list the HEAD version of the documentation pages to avoid page duplicates from being listed due to support of multiple document versions.
- Fix the col2 class so it clears the float and the footer can be displayed correctly in tablet landscape mode. Add some margins to form a gutter in between columns.
- Enhance the default.html to only include Doxygen related CSS and Javascript when the page is a documentation page.
- Fix a potential problem due to post entry having long list of tags which breaks the rendering on a extra small (iPhone) viewport.
- Ensure the images and other embedded medias in the <article> does not exceed predefined max-width.
- Change nil layout to null layout to avoid warning messages from newer Jekyll version.