Remove mathjax from repository

Release !67 (merged) must be merged first.

This PR removes the big folder "Mathjax" from the documentation.

The rationale of embedding mathjax into the repository was to avoid making numerous requests to external servers, even when browsing a local documentation. However, we end up having 16 Megabytes of javascript in the repository, which is not satisfactory.

With this PR, a compressed Mathjax installation is fetched from the silx server and automatically decompressed at build time.