Fixes #409

Add documentation for `pytest-recording` and VCR cassettes in `CONTRIBUTING.md`.

* Add a section for `pytest-recording` plugin.
* Link to `pytest-recording` documentation.
* Provide a brief summary on how to record a new VCR using `uv run pytest tests/desired_test_module.py`.
* Link to the config in `tests/conftest.py`.
* Document header removals from responses.
* Add relative links for `tests/cassettes` and `tests/conftest.py`.
* Add a comment about keeping the cassettes less than 1 MB in size.

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For more details, open the [Copilot Workspace session](https://copilot-workspace.githubnext.com/Future-House/paper-qa/issues/409?shareId=XXXX-XXXX-XXXX-XXXX).

…2) it restated the plugin's default from https://github.com/kiwicom/pytest-recording/blob/v0.13.2/src/pytest_recording/plugin.py#L73-L76