Fix readthedocs build error

[jcfr]

From https://app.readthedocs.org/projects/vtk-book/builds/27309435/

Build output directory contains multiple files Build output directory for format "pdf" contains multiple files and it is not currently supported. Please, remove all the files but the "pdf" you want to upload.

This issue is independent of #18

[vibraphone]

@jcfr Would this be resolved by not generating a PDF (HTML only)? Should we do that?