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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

Remove mesos lib from pyspark-notebook

upgrade jupyterlab to 2.1.3 to avoid npm delay

The best links connects a linking object and its text occurrence, e.g. the link to documentation is near documentation's occurrence in the text. Since no explicit link to the guide the guide text font became bold but link to the docs moved to doc's text occurence

Print image size after building it