Start using Ubuntu 20.04 (focal)

Fix bash files codestyle

Fix yaml files codestyle

Docker linting -> Hadolint

Improve documentation

Follow-on PR #911: Spark documentation rework

* Test added for all kernels
* Same examples as provided in the documentation (`specifics.md`)
* Used the same use case for all examples: sum of the first 100 whole numbers

Note: I've not automatically tested `local_sparklyr.ipynb` since it creates by default the `metastore_db` dir and the `derby.log` file in the working directory. Since I mount it in `RO` it's not working. I'm struggling to set it elsewhere...

Some changes to the Spark documentation
for local and standalone use cases with the following drivers

* Simplify some of them (removing options, etc.)
* Use the same code as much as possible in each example to be consistent (only kept R different from the others)
* Add Sparklyr as an option for R
* Add some notes about prerequisites (same version of Python, R installed on workers)

CONTRIBUTING.md misleading link to guide

doc/index.rst : more readable Issue Tracker's link on sidebar