Move documentation to Read The Docs

[fepegar]

Now that InnerEye actually has Read The Docs (http:https://innereye-deeplearning.rtfd.io/), that's probably where all the documentation should go (as opposed to markdown files in this repository).

Symlinks work, so files wouldn't need to be moved around, just linked.

AB#6514

[peterhessey]

Picking this up, however as per discussion with @fepegar yesterday docs files will be moved around in order to have only a single documentation folder.