Moving imstk documentation to readthedocs

imstk’s documentation is now available on readthedocs.org

You can access the documentation at the links below

Stable:
https://imstk.readthedocs.io/en/stable/

latest:
https://imstk.readthedocs.io/en/latest/

with release tag TAG
https://imstk.readthedocs.io/en/TAG/

The documentation build and update are triggered automatically after every merge