Add Sphinx documentation

[rk1a]

• Adds dev tools: pre-commit, isort, black, flake8
• Adds doc strings to minetester python package
• Adds basic Sphinx documentation of Minetester features
• Makes dumb client create a recorder by default (without passing --record)
• Fixes segfault when not passing cursor image
• Updates README with documentation build instructions

Related issues: #43 #19

Ready for Review.

How to test

pip install -e .[docs]
cd docs-minetester && make livehtml