Improvements in documentation automated build

This MR bumps Sphinx version from unreleased branch 4.x and replaces numpydoc extension with sphinx.ext.napoleon.

This is connected with !386 (closed) development, but I decided to make it a separate MR, since it alters Docker images that are global for all branches.

The reason to replace current Sphinx with git version is that on current stable (4.0.2) only properties are documented in "Attributes" rubric:

while on unreleased 4.1 attributes are also captured correctly: