gh-119786: move 'changing grammar' checklist from devguide to InternalDocs (#125874)

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diff --git a/InternalDocs/README.md b/InternalDocs/README.md
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--- a/InternalDocs/README.md
+++ b/InternalDocs/README.md
@@ -19,6 +19,8 @@ Compiling Python Source Code
 
 - [Compiler Design](compiler.md)
 
+- [Changing Python's Grammar](changing_grammar.md)
+
 Runtime Objects
 ---
 
diff --git a/InternalDocs/changing_grammar.md b/InternalDocs/changing_grammar.md
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@@ -0,0 +1,63 @@
+# Changing CPython's grammar
+
+There's more to changing Python's grammar than editing
+[`Grammar/python.gram`](../Grammar/python.gram).
+Below is a checklist of things that may need to change.
+
+> [!NOTE]
+>
+> Many of these changes require re-generating some of the derived
+> files. If things mysteriously don't work, it may help to run
+> ``make clean``.
+
+## Checklist
+
+* [`Grammar/python.gram`](../Grammar/python.gram): The grammar definition,
+  with actions that build AST nodes.
+  After changing it, run ``make regen-pegen`` (or ``build.bat --regen`` on Windows),
+  to regenerate [`Parser/parser.c`](../Parser/parser.c).
+  (This runs Python's parser generator, [`Tools/peg_generator`](../Tools/peg_generator)).
+
+* [`Grammar/Tokens`](../Grammar/Tokens) is a place for adding new token types.  After
+  changing it, run ``make regen-token`` to regenerate
+  [`Include/internal/pycore_token.h`](../Include/internal/pycore_token.h),
+  [`Parser/token.c`](../Parser/token.c), [`Lib/token.py`](../Lib/token.py)
+  and [`Doc/library/token-list.inc`](../Doc/library/token-list.inc).
+  If you change both ``python.gram`` and ``Tokens``, run ``make regen-token``
+  before ``make regen-pegen``.
+  On Windows, ``build.bat --regen`` will regenerate both at the same time.
+
+* [`Parser/Python.asdl`](../Parser/Python.asdl) may need changes to match the grammar.
+  Then run ``make regen-ast`` to regenerate
+  [`Include/internal/pycore_ast.h`](../Include/internal/pycore_ast.h) and
+  [`Python/Python-ast.c`](../Python/Python-ast.c).
+
+* [`Parser/lexer/`](../Parser/lexer/) contains the tokenization code.
+  This is where you would add a new type of comment or string literal, for example.
+
+* [`Python/ast.c`](../Python/ast.c) will need changes to validate AST objects
+  involved with the grammar change.
+
+* [`Python/ast_unparse.c`](../Python/ast_unparse.c) will need changes to unparse
+  AST involved with the grammar change ("unparsing" is used to turn annotations
+  into strings per [PEP 563](https://peps.python.org/pep-0563/).
+
+* The [`compiler`](compiler.md) may need to change when there are changes
+  to the `AST`.
+
+* ``_Unparser`` in the [`Lib/ast.py`](../Lib/ast.py) file may need changes
+  to accommodate any modifications in the AST nodes.
+
+* [`Doc/library/ast.rst`](../Doc/library/ast.rst) may need to be updated
+  to reflect changes to AST nodes.
+
+* Add some usage of your new syntax to ``test_grammar.py``.
+
+* Certain changes may require tweaks to the library module
+  [`pyclbr`](https://docs.python.org/3/library/pyclbr.html#module-pyclbr).
+
+* [`Lib/tokenize.py`](../Lib/tokenize.py) needs changes to match changes
+  to the tokenizer.
+
+* Documentation must be written! Specifically, one or more of the pages in
+  [`Doc/reference/`](../Doc/reference/) will need to be updated.