Add sphinx docs folder and changelog URLs to metadata (#751)

docs/subprocess.rst

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+Running tests in a Python subprocess
+====================================
+
+To instantiate a ``python3.9`` subprocess and send tests to it, you may type::
+
+    pytest -d --tx popen//python=python3.9
+
+This will start a subprocess which is run with the ``python3.9``
+Python interpreter, found in your system binary lookup path.
+
+If you prefix the --tx option value like this::
+
+    --tx 3*popen//python=python3.9
+
+then three subprocesses would be created and tests
+will be load-balanced across these three processes.