Add documentation to docker images

This MR adds documentation to the nektarpp/nektar docker image, including PDFs of the user guide, developer guide and tutorials. A number of tweaks have been added to the main repository and the tutorial repository to enable this:

• Install hooks added to install files when they are built through e.g. make tutorials.
• Some issues fixed in Doxyfile, and redundant per-library Doxyfile removed.
• Switched Doxyfile to produce SVGs (better quality graphs) and removed graphs for include hierarchy/collaborations as these were rather large and unhelpful in most cases.
• Tutorial subrepo URL has been moved to https vs git to enable checkout on builders.
• Missing pstricks dependency added to developer guide documentation.