Mostieri/improve docs

[mariostieriansys]

Some improvements to docs:

1. Brief introduction to EnSight and PyEnSight, pointing to the relevant docs and websites.
2. I copy/pasted the extension docs from the PyEnSight documentation
3. I removed the badges pointing to the PyEnSight actions
4. I also removed the PyPi badge, since as a first cut we don't need to publish the wheel into PyPi anymore