* added notes re literal_include to python examples
* merged changes in #1504 
* deduped and includified (https://github.com/arbor-sim/arbor/pull/1558) tutorials.
* First divergence of tutorial and code spotted!
* Added some contrib documentation for examples and docs.

- Minor fixes to python examples.
- Fix `mpi.py` to use new label API.
- Rename `mpi.py`.

* Tutorial ring network added. A few typos fixed, crosslinks fixed.
* MPI tutorial added.
* Improved SVG generation, added separate label scaling, increased default label size.
* Bugfix: didnt generate cable region in gen-labels.py used in `concepts/morphology.rst`.
* Termified Interconnectivity page.
* Termified probe nomenclature page. Will be moved in [this PR](https://github.com/arbor-sim/arbor/issues/1353) from python/simulation to concepts/probes_samplers.