Created developer instructions

[turian]

A few changes:

• 'dev' is an extra optional requirement, installing things like pytest and pytest-cov.
• Documentation is cleaned so that a) docs go straight at the top (it would be nice in the sidebar too but you have perms to do that; I didn't even know they exist); b) There is a separate section on how to get starting doing dev on solo-learn. (I wanted to try adding some patches but can't without this basic tooling to run easily.)
• One thing you might consider is having separate dev and test requirements. You can see here that a lot more dependencies are required for dev than test. But many people (like github CI) just want test deps.

[codecov[bot]]

Codecov Report

Merging #302 (4998bfa) into main (701776d) will not change coverage. The diff coverage is n/a.

Flag	Coverage Δ		*Carryforward flag
cpu	81.68% <ø> (ø)
dali	40.06% <ø> (ø)		Carriedforward from a12bf92

*This pull request uses carry forward flags. Click here to find out more.