Convert README to Markdown

[Mirochill]

Fixes #61.

Summary

• replace the reStructuredText README with a GitHub-flavored Markdown README
• preserve the existing sections, links, command examples, and directory tree in Markdown syntax
• remove README.rst after adding README.md

Validation

• git diff --check HEAD~1..HEAD
• git show --stat --summary --oneline --name-status HEAD
• rg -n "^# |^## |Cookiecutter|Briefcase|WiX|Python for \.NET|Toga|README\.rst" README.md
• Not run locally: tests, build, pre-commit, documentation renderer, install, or project commands.

PR Checklist:

• I will abide by the BeeWare Code of Conduct
• I have read and have followed the CONTRIBUTING.md file
• This PR was generated or assisted using an AI tool

Assisted-by: OpenAI Codex

[Mirochill]

Closing this after reviewing BeeWare's AI Policy. I missed the org-level AI contribution requirements before opening the PR; the policy requires disclosure and says unsolicited PRs from autonomous tools should not be submitted without prior permission. I won't pursue this PR further.

[freakboy3742]

Closing this after reviewing BeeWare's AI Policy. I missed the org-level AI contribution requirements before opening the PR; the policy requires disclosure and says unsolicited PRs from autonomous tools should not be submitted without prior permission. I won't pursue this PR further.

You've misread the AI contribution policy. The AI contribution policy says nothing about prior permission. It only mentions disclosure and human responsibility.