Address medium priority issues

[jdebacker]

Summary

This PR expands several medium-priority parts of the Git tutorial and improves the repository’s contributor-facing documentation.

The main goals were to:

• turn placeholder chapters into usable beginner-facing guidance
• improve setup documentation for new Git and GitHub users
• correct and modernize the Git cheat sheet
• align top-level build instructions with the repo’s newer Jupyter Book / MyST setup

What changed

Repository management chapters

Expanded the following chapters from short placeholders into real content:

• book/content/repomgt/testing_CI.md
• book/content/repomgt/virt_env.md
• book/content/repomgt/license.md
• book/content/repomgt/documentation.md

These now cover:

• CI vs unit tests vs regression tests
• practical virtual environment choices (conda, venv, and Docker context)
• open-source licensing basics and private repo considerations
• documentation structure, contributor docs, and README expectations

Beginner setup chapters

Expanded the following setup chapters:

• book/content/using/git_config.md
• book/content/using/installing_git.md
• book/content/using/create_GitHub_acct.md

These now include more concrete guidance on:

• checking whether Git is installed
• configuring user.name, user.email, editor, pull strategy, and default branch
• GitHub account setup, authentication expectations, and basic account hygiene

Cheat sheet fixes

Updated:

• book/content/using/GitCheatSheet.md

This fixes misleading examples and improves the command list by:

• replacing the old merge-direction examples with clearer main / feature-branch flows
• adding git diff and git diff --staged
• including git pull --ff-only
• using clearer descriptions for staging, committing, and updating from upstream

README updates

Updated:

• README.md
  This refreshes the build instructions to better match the repo’s current Jupyter Book / MyST direction and removes some outdated framing.

Notes

• This branch builds on top of the earlier codex/high-priority-tutorial-pass tutorial improvements.