Define developer environment

[redeboer]

Originally posted in #1 (review)

Targets:

1. Make it easier to set up a (reproducible) virtual environment for this repo after cloning
2. Add configuration files for linters and editors.
3. Enforce automated style checks on PR through pre-commit.ci
4. Add Sphinx documentation infrastructure
5. Add GitHub workflows, especially for GitHub Pages

To do

Beta Give feedback

1. docs/conf.py (remove the options that are not yet needed) and a skeleton index.md for the main page
2. Empty src/kmatrix/__init__.py source code module (can be extended later)
3. Empty tests/__init__.py folder. This is new, but you can use it later to 'pin' framework behaviour with unit tests. Note that you have already configured pytest.
4. .editorconfig for EditorConfig
5. .envrc for activating environment with direnv
6. .gitignore
7. .prettierignore (mainly there fore VS Code)
8. .taplo.toml for TOML formatting
9. .vscode/settings.json and recommended .vscode/extensions.json
10. LICENSE
11. .github/workflows
12. Install pre-commit.ci into this repo (has to be done via that website)