docs: Document system dependencies and clarify tool usage

[rgant]

Summary

Documents system dependencies required to run the test suite, which are currently undocumented and can cause confusing build failures for new contributors.

Changes

• New "System Dependencies" section covering:

  • MySQL/MariaDB client libraries (required for mysqlclient to build)
  • GDAL and GEOS (optional, for django.contrib.gis tests)
  • Platform-specific instructions for Ubuntu/Debian, Fedora/RHEL, and macOS (Homebrew)
  • Note about macOS library path issues and workaround for GIS tests

• Pre-commit clarification: Added note that pre-commit must be installed separately, with link to installation options and mention of uvx alternative.

• uv run pytest: Changed from pytest -n auto to uv run pytest -n auto for consistency with uv-based workflow (works regardless of venv activation).

Motivation

When setting up the dev environment on macOS, uv sync fails with a cryptic mysqlclient build error unless you have MariaDB/MySQL client libraries installed. Similarly, 2 GIS tests fail without GDAL/GEOS.
These dependencies aren't mentioned anywhere in the docs.

This PR documents what I learned while setting up the project for the first time.

Test plan

• pre-commit run --files CONTRIBUTING.md passes
• Verified instructions work on macOS with Homebrew

[rgant]

I'm working on a real PR, but first I wanted to get this setup. So I documented my process as a fresh contributor to ensure the documentation is robust.

I normally format markdown files, but that will introduce a lot of noise so I didn't include it. If you are interested I would be happy to contribute a dprint setup to format markdown files for the project automatically.

[ngnpope]

Can we not just point to the documentation already in Django to ensure that we stay in sync?

• https://docs.djangoproject.com/en/6.0/topics/install/#how-to-install-django
• https://docs.djangoproject.com/en/6.0/internals/contributing/writing-code/unit-tests/#running-all-the-tests
• https://docs.djangoproject.com/en/6.0/ref/contrib/gis/install/geolibs/#installing-geospatial-libraries

Another helpful hint as far as Debian/Ubuntu packages is this list:

• https://github.com/django/django-docker-box/blob/main/packages.txt

[rgant]

Thanks for the feedback! I've updated the section to link to external docs for the install details:

• MySQL/MariaDB C libraries now link to the https://github.com/PyMySQL/mysqlclient#install (Django's own docs present this as optional, so they don't cover the system-level prerequisites)
• GDAL/GEOS now links to Django's https://docs.djangoproject.com/en/stable/ref/contrib/gis/install/geolibs/ page
• Added the https://github.com/django/django-docker-box/blob/main/packages.txt as a reference for Debian/Ubuntu

I kept the macOS GDAL/GEOS symlink workaround since that isn't covered in any upstream docs and is a real stumbling block for MacOS contributors. I also kept a brief note explaining that these dependencies are
required here even though Django treats them as optional — a contributor coming from Django might not expect to need them. I did not.