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

Author: iritkatriel

InternalDocs/index.md

@@ -0,0 +1,12 @@
+
+# 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).