Enhancement: Dropdown menu to select OpenCL version

As already briefly noted [here](https://github.com/KhronosGroup/OpenCL-Docs/issues/251#issuecomment-633670561):

Is it possible, to add a drop-down-menu to the header of all reference pages (old ones and new ones), that lets one switch to a newer or older version?

Alternatively, could at least a link to the latest version be added to all older versions?

Start by reviewing the shared headers on the OpenCL reference pages, including both old and new versions, and determine how version links are currently represented. Compare the dropdown and latest-version-link alternatives; done means users can move between available versions from every reference page, or older pages consistently link to the latest version.

Written by the indexing model from the issue text.