TEST: Wire docstring doctests into the unittest suite

[christianescamilla15-cell]

Summary

Closes #473.

• Adds test/core/doctest_test.py which uses the unittest load_tests protocol and doctest.DocTestSuite to run docstring examples as part of the standard test job. Modules with doctests are listed explicitly in MODULES_WITH_DOCTESTS so the runner does not force-import heavy optional dependencies (LAL, gwpy, samplers) in minimal environments.
• Converts the existing example in bilby.core.utils.introspection.infer_args_from_function_except_n_args from a .. code-block:: python directive (which doctest ignores) into a real doctest-format example, so the new runner has a first test to exercise.
• Documents the workflow for adding new doctests in CONTRIBUTING.md.

Rationale: the issue asks for automated discovery of docstring examples and points at the unittest + doctest integration, so using the load_tests hook keeps discovery automatic and integrates cleanly with the existing pytest test invocation without requiring pytest --doctest-modules (which would force-import every bilby submodule).

Test plan

• Verified the new doctest format parses and runs with doctest.DocTestSuite on a representative module.
• Added sanity-check tests (TestDoctestInfrastructure) confirming that MODULES_WITH_DOCTESTS is populated, all listed modules import cleanly, and the introspection module reports at least one doctest to the DocTestFinder.
• Compiled test/core/doctest_test.py with python -m py_compile.
• CI green on bilby-dev CI.