Builds the docs on the target server

With this MR, the docs are built on the remote side, which has the following advantages:

• The docs are always up to date (instead of copying what is in /site)
• Less dependencies on the local machine (no need to install mkdocs locally)
• The built documentation is no longer versioned

It is still possible, should you so wish, to build the docs locally, I've left the relevant make target.