pythongh-119786: create folder in cpython repo for internals documentation

Author: iritkatriel

InternalsDoc/index.md

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+# CPython Internals Documentation
+
+The documentation in this fodler is intended for CPython maintainers.
+It described implementation details of CPython, which should not be
+assumed to be part of the Python language specification. These details
+can change between any two CPython versions and should not be assumed
+to hold for other implementations of the Python language.
+
+The core dev team attempts to keep this documentation up to date. If
+this is not the case, please `create an issue <https://github.com/python/cpython/issues>`__.
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+
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+