gh-119786: create folder in cpython repo for internals documentation (#119787)

Author: iritkatriel

InternalDocs/index.md

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+
+# CPython Internals Documentation
+
+The documentation in this folder is intended for CPython maintainers.
+It describes 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
+it is not, please report that through the
+[issue tracker](https://github.com/python/cpython/issues).