Add GitHub Copilot instructions

[millsks]

Description

Add a .github/copilot-instructions.md file that gives GitHub Copilot useful background about this repository as a Cookiecutter template. The guidance will explain that the code under {{cookiecutter.project_slug}}/ uses Jinja2, outline how the template is rendered and how hooks can add or remove files, and call out the difference between tests run against the template and tests run in generated projects. It will also include notes on the common Docker commands, dependency workflows, and recommended patterns for implementing optional features. Finally, add a short pointer in CONTRIBUTING.md so contributors know the guidance exists.

This is documentation only. It will not change code, CI, or the files that end up in generated projects, and it only affects users who enable Copilot or who read the file directly.

Checklist:

• I've made sure that tests are updated accordingly (especially if adding or updating a template option)
• I've updated the documentation or confirm that my change doesn't require any updates

Rationale

The repository uses a templating system and hook scripts that are easy to misunderstand. Without that context, AI code suggestions and some contributor edits can accidentally break Jinja2 syntax or ignore the hook and testing conventions. Providing clear, repo-specific instructions helps Copilot offer safer, more accurate suggestions and helps contributors avoid common mistakes. This is a small, low-risk improvement that makes the project easier to work with for people who use Copilot, and it doubles as a handy reference for anyone learning the template structure.

Resolves #6207

[millsks]

This is only a starting point for this file. If this file gets merged I would love to see the suggestions and modifications from the cookicutter-django community!

[browniebroke]

I'm not too keen on adding agent-specific instructions to the repo because this will open the door to add others (Cursor rules, Claude-specific file, Codex, Jules, ...). This will mean creating a number of files with almost the same content.

If we add these kind of instructions, I'd rather use something like AGENTS.md, which seems to be the more standard way of doing that, and is supported by many.