DOC: Add all `InnerEye/ML` docstrings to ReadTheDocs

[peterhessey]

Closes #774

Adds all the docstrings in InnerEye/ML to the ReadTheDocs site.

Includes many corrections to docstrings formatting to ensure that they render correctly.

Apologies for the size of PR, but the majority of the line changes are whitespace erro and reStructuredText formatting corrections that I automated to save time. The critical changes for review are all found in the docs/ directory. i.e. the .rst files.

[peterhessey]

The sphinx-build process is still dumping a million warnings currently. This will be fixed in a separate PR - I think that a lot of these will disappear when we switch to a new reStructuredText parser.