make deploy_doxygen CI step checkout submodules

This MR adds the following line to the variables field in the deploy_doxygen step in the CI:

GIT_SUBMODULE_STRATEGY: recursive

The CI step deploy_doxygen is failing on branch devel since the cmake3 command fails due to the checks for non-empty directories in the /exts/ directory (checked here).

Even though the sub-modules are not needed for the doxygen build, these directories are already excluded by the doxygen configuration here and so there is no worry about accidentally building whatever doxygen is in the external packages.