added GitHub Codespaces setup instructions to CONTRIBUTING.md

[suiiibhit]

Closes: #291
Issue number of the reported bug or feature request: #291

Describe your changes
Added a "Setting up a development environment" section to CONTRIBUTING.md
documenting how to use GitHub Codespaces as well as local setup steps.
Also expanded the dev container's onCreateCommand to install all contributor
dependencies (requirements-test.txt, requirements-extra.txt) and configure
pre-commit hooks automatically on container creation.

Testing performed
Verified the dev container configuration is valid JSON and that the
onCreateCommand installs all required dependencies for running tests and
linting. Confirmed no source files were modified.

Additional context
The .devcontainer/devcontainer.json already existed with the correct ptrace
capabilities (--cap-add=SYS_PTRACE, seccomp=unconfined) needed for pystack
to work inside a container. This change ensures Codespaces is fully usable
out of the box and that contributors know it exists.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.19%. Comparing base (178a765) to head (c7c3619).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #306   +/-   ##
=======================================
  Coverage   79.19%   79.19%           
=======================================
  Files          51       51           
  Lines        5561     5561           
  Branches      583      583           
=======================================
  Hits         4404     4404           
  Misses       1157     1157           

Flag	Coverage Δ
cpp	79.19% <ø> (ø)
python	79.19% <ø> (ø)

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[sarahmonod]

This looks great! Thanks for contributing to our open source project. I think a few small things can be improved though, can you please take a look and tell me what you think of them?

[sarahmonod]

LGTM, thanks @suiiibhit!

[sarahmonod]

Ah woops, it looks like the dco wasn't added. Can you please run git commit --amend -s @suiiibhit ?

[godlygeek]

Looks good to me as well, though I reverted the changes to reference requirements-test.txt and requirements-extra.txt in devcontainer.json - the test and extra dependencies are already installed by the Dockerfile that the devcontainer builds on top of, so adding those to the install was redundant, and it conflicts with #296, which removed those files in favor of using dependency groups.