Document usage of just command line runner

[kaimen-sano]

Description and Motivation

In #274 we introduced an env loader for the CLI to simplify and unify our commands used in the repository. This commit documents the usage of the just command runner for new users of the repository.

---

Checklist:

• [X] I have read Migaloo's contribution guidelines.
• [X] My pull request has a sound title and description (not something vague like Update index.md)
• [X] All existing and new tests are passing.
• [X] I updated/added relevant documentation.
• [ ] The code is formatted properly cargo fmt --all --.
• [ ] Clippy doesn't report any issues cargo clippy -- -D warnings.
• [ ] I have regenerated the schemas if needed cargo schema.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (b751c49) 94.46% compared to head (49c6655) 94.46%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #283 +/- ## ======================================= Coverage 94.46% 94.46% ======================================= Files 219 219 Lines 24858 24858 ======================================= Hits 23482 23482 Misses 1376 1376 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.