docs: remove setup.py mentions, mention presets

[henryiii]

Description

Update contributing / changelog with some updates related to build systems.

---

📚 Documentation preview 📚: https://pybind11--5677.org.readthedocs.build/

[Copilot]

Pull Request Overview

This PR updates documentation to remove references to setup.py and introduce CMake presets and nox-based workflows.

• Remove mentions of setup.py in CMakeLists and CONTRIBUTING guide
• Add CMake presets section and update test/venv commands in CONTRIBUTING.md
• Update changelog to reflect removal of PYBIND11_GLOBAL_SDIST

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
docs/changelog.md	Update entry to note removal of PYBIND11_GLOBAL_SDIST
CMakeLists.txt	Remove outdated setup.py comment and align header management note
.github/CONTRIBUTING.md	Replace setup.py commands with CMake presets and add presets section

Comments suppressed due to low confidence (2)

.github/CONTRIBUTING.md:28

• The guide uses --workflow but later examples use --preset. Consider standardizing on cmake --preset venv (or clarify what --workflow means) for consistency.

* Add tests for any new functionality and run the test suite (`cmake --workflow venv`)

CMakeLists.txt:181

• [nitpick] Since setup.py is no longer used, reintroduce a comment to remind maintainers to update the CMakePresets.json (or equivalent) when adding new headers.

-# NB: when adding a header don't forget to also add it to setup.py