pip-freeze: add page (#12184)

pages/common/pip-freeze.md

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+# pip freeze
+
+> List installed packages in requirements format.
+> More information: <https://pip.pypa.io/en/stable/cli/pip_freeze>.
+
+- List installed packages:
+
+`pip freeze`
+
+- List installed packages and write it to the `requirements.txt` file:
+
+`pip freeze > requirements.txt`
+
+- List installed packages in a virtual environment, excluding globally installed packages:
+
+`pip freeze --local > requirements.txt`
+
+- List installed packages in the user-site:
+
+`pip freeze --user > requirements.txt`
+
+- List all packages, including `pip`, `distribute`, `setuptools`, and `wheel` (they are skipped by default):
+
+`pip freeze --all > requirements.txt`