docs: fix CONTRIBUTING claims and add the missing CODE_OF_CONDUCT

[Avicennasis]

Closes #48954, #48955, #48956. Removes the phantom python-dotenv dependency, corrects the CI Python version (3.14, not 3.12), and adds the CODE_OF_CONDUCT.md that CONTRIBUTING already referenced.

[Avicennasis]

The two stated corrections are accurate and the Code of Conduct link resolves, but the documented setup still fails in a fresh virtualenv: it runs pre-commit install without installing pre-commit. Add pre-commit to the install command (preferably pin all dev tools through a requirements/pyproject file rather than a floating manual list).

Also, python-dotenv remains installed by .github/workflows/test.yml even though the PR correctly identifies it as a phantom dependency. Remove it there too or avoid claiming the dependency has been removed from the project setup. All current GitHub checks and git diff --check are green.

[Avicennasis]

Re-review at 1fd8a84: added the missing pre-commit install, pinned the documented Ruff version to the workflow/config 0.15.22, and removed the phantom python-dotenv CI install so docs and automation agree. Workflow YAML parses; Ruff check/format, 40 tests, and diff checks pass locally. Fresh full GitHub CI is running.