Initial 'doc-switcher' implemented using Liquid tags and BS's dropdown.
The links in the dropdown are statically generated when the site is rebuilt.
Current drawback: newer page that is only available at higher version when being switched to lower version may result in 404. This drawback could be resolved using Liquid tags to check the site.pages array for its availability and adding a 'disabled' class when not available. However, this part of the logic is not checked in because otherwise the site rebuild would take much longer time to complete.

Other changes:
- Better sentinel/guard for group exclusion.
- Match the dropdown link color with orderedlist/minimal theme.