Add Developer Documentation

[thorstenhater]

Introduction

To onboard new devs we need to able to get them up to speed quickly.
Here's my (rough) proposal

• Dev docs exist next to user docs, potentially touching similar topics.
• In depth information on topic should be in the relevant (dev / user) part and cross-referenced.
• Articles in doc/internal should be integrated into the developer part.
• Dev docs should be structured analoguous to the data flow.

I start in this PR by adding an introductionary guide to cable cells. It outlines the flow
of cable cell simulations through arbor, sketches how cable cell simulations are set up,
and has jumping off points to more in depth topics.

TODO

• Images
• Backends
• In-depth guides
  • mech-abi (can be migrated from internal)
  • backends
    • gpu
    • cpu (SIMD be migrated from internal)
  • shared state
  • event handling
  • solvers
  • discretisation
  • cell groups/super-cells/load balancing
• Theory
  • cable equation (pull from cable_report.tex)
  • Numerical methods

[brenthuisman]

I've added the script+css that can be used to generate diagrams. Preview on the Concepts>{index,recipe} pages: https://arbor-rtd-dev.readthedocs.io/en/latest/

Needs to be expanded to cover all Concepts pages that benefit from a visual representation of their context.

[noraabiakar]

This is a really good start, thanks a lot. I have a few comments and suggestions.

[brenthuisman]

• I've given the schematics a visual update (bit more compressed), made them foldable inplaces and moved them to more appropriate places.
• internals merged into dev. I've hidden the numerical methods page for now, as it did not really contain anything. That OK?
  • Update: I see now that this is a placeholder for doc/math, which has updates in this branch: https://github.com/halfflat/arbor/commits/doc/update-model I say we merge those changes in this PR, and link to the TeX for the time being.

[brenthuisman]

• Our beloved graphviz is back: a copy of the Arbor dep graph, and some more on a new Ecosystem page. Includes a list of all Arbor tools (such as nmlcc). Yes, not great, because graphviz and great plots are incompatible, but very helpful to have nonetheless.
• Small change to bring some order to the tutorials as per Documentation: tutorial structure, howtos #1369.

[brenthuisman]

The only change I'd want to put in here (besides corrections): dev/numerics.rst content and/or https://github.com/halfflat/arbor/tree/doc/update-model merged.

Possibly by using https://github.com/michael-brade/LaTeX.js

[brenthuisman]

There is one last issue, I can't get RTD to generate the graphviz plots (which I'm certain I could a year ago or so).