Static files

[jorshi]

We should decide on the main location for static files. For images it seems like the jupyter book way is to use a folder called images -- however, I ran into a bit of trouble trying to include rendered video files which I included in an HTML video tag. These files were not automatically copied into the build folder by sphinx. The sort of hacky solution was to put these into a folder called _static, which happens to be copied by sphinx build. I think this is the only way to support videos, so I went with the approach for all static files in my PR #4

[rodrigodzf]

I saw the same problem with an audio file just now. For most of the physical modelling part, I generated and rendered the videos within the notebook, which worked fine; however, this isn't a solution for all media content. It is probably better to address the issue once we have merged the rest of the branches.