* Adapt CSS for the HTML5 transition in docutils 0.17
* Fix some reST markup errors
* Add meta-tag to enable indexation control at Google (current indexation appears at least a year old).

Wrap place_pwlin interface to allow querying rich geometry information from cell morphologies.

* Add Python bindings for the `place_pwlin` and `isometry` classes.
* Add equality test for Python `mpoint`.
* Add unit tests for new Python interfaces.
* Split C++ API morphology documentation into its own file.
* Add C++ API and Python API documentation for `place_pwlin` and `isometry`.

* Synced pages between Concepts, Python API and C++ API wherever possible
    * Recipe pages conform between the three section (concepts, python, c++)
    * Cell, Cable Cell and Cell * pages are rearranged and provided with some copy explaining the relationship between them.
* Moved Python API out of Concepts
* Renamed Concepts "How does Arbor work?"
* Added Python Module Index plus mock import of Arbor for RTD build (unfortunately won't show there)
* Broke out Interconnectivity (synapses) page.
* Reworked Single Cell Model page into a quick start, with lots of cross referencing.
* Tweaked logo.
* Added Spack to install options.
* Updated blurb.
* Documentation now follows EU capitalization rules.
* Assorted typofixes

This PR adds a bunch of high-level docs for concepts related to single cell models.
  * morphologies
  * labels (region and locset expressions)
  * mechanisms
  * cable cells

It adds some python scripts to the `doc` path, which are called during documentation building to generate images of cell morphologies, regions and locsets that are generated in a separate script that uses the Arbor Python API.

The docs are not complete: the cable cell and mechanism docs are unfinished, and the tutorial section needs to build on these concepts to give more detailed step by step examples.