In order to be able to control better which branches are actually used
for building the docs, this commit changes the logic of the script to
only add manually specified versionsi to the docs.
This commit contains:
* fix the build script, so that only versions are added that have a
`docusaurus.config.js`
* some refactoring so that the build script will work well with github
pages
* add configuration to the `docusaurus.config.js` to display a dropdown
menu to select a version
The behavior for the versioning is now so that 2.6 is both the 'latest'
version as well as the 'next' version. As soon as 2.7 will be built that
will be displayed as the 'next' version.
closes#16671
