Add `InnerEye/ML` docstrings to ReadTheDocs

[peterhessey]

Is there an existing issue for this?

• [X] I have searched the existing issues

Issue summary

The IE-DL API for Read the Docs needs to be completed. Currently only a small subset of the code is included, and the stuff that is included produces many warnings during the build process that we should ideally be avoiding.

What documentation should be provided?

All Docstrings in InnerEye/ML should be added to the ReadTheDocs site.

AB#6801

[peterhessey]

Look into using the sphinx autosummary tool for this.