Update docs and bump version for 9.4.0 release

"Version history" has been moved under "What's new". The "What's new" section has a quick overview, and then links to a detailed migration guide, and the existing version history.

Getting started was improved, with a focus on installing the binary packages. And a few other small updates.

Changes best viewed here: https://pytango.readthedocs.io/en/update-docs/ (an unpublished version on rtd).

Version is also bumped for release.

Related: #491 (closed)