The link in `WIDGET_HELP_PATH` is outdated

WIDGET_HELP_PATH points to http://www.silx.org/pub/doc/est/.htmlhelp/latest/ which only contains releases up to 0.3.

We can either:

• Upload latest releases to silx.org
• Forego documentation on silx.org and use ReadTheDocs to version our documentation instead since we already have the RtD integration (https://ewoksest.readthedocs.io/en/latest/). In this case, the link of WIDGET_HELP_PATH should be changed to https://ewoksest.readthedocs.io/en/latest/.

Thoughts @payno @denolf ?