Convert Singularity material into standalone Docker+Singularity workshop

[liz-is]

• update setup instructions for our system
• add docker-intro folder download to setup page
• rework introduction.md
• remove redundant material from 01-singularity-gettingstarted.md
• integrate material from 05-singularity-docker.md and docker-hub.md
• update singularity version number to something more recent
• add a callout noting the Docker equivalents of singularity exec, singularity run, singularity shell
• fix link in singularity-files.md
• add info on Singularity Hub to docker-hub.md
• add an exercise on binding directories with Singularity
• rework advanced-containers.md to reduce focus on binding directories with Docker as this has already been covered with Singularity only made minor changes, as the content is helpful for flow
• add section/exercise about pulling the image you made with Singularity
• MPI: remove section on building image with a .def file and provide pre-built .sif instead
• edit wrap-up section and reproducibility page to mention Singularity

[liz-is]

@jag1g13 I think this is pretty much done apart from the MPI episode! I haven't updated the episode timings but they are just an estimate anyway... there's probably things I've missed so let me know if you spot anything that needs updating.

Some thoughts on potential further changes after running through it myself:

• move section on Docker mounts earlier, before we introduce Singularity?
• add more exercises?
• I added a callout with a comparison of key Docker and Singularity commands, but maybe a separate reference page would be better and could be more comprehensive

Let me know if you want me to do anything for adapting the MPI section!