Document how to add a bytecode specialization

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How to add a new bytecode specialization is knowledge only a few of us remember right now. We should document this somewhere (devguide or CPython, I don't mind). Hopefully a new contributor can pick this up!

There reference is this PR: https://github.com/python/cpython/pull/116385/files

Assuming you found an instruction that serves as a good specialization candidate. Let's use the example of CONTAINS_OP:

1. Set the original CONTAINS_OP to a uop by changing it from CONTAINS_OP to _CONTAINS_OP in Python/bytecodes.c. The instruction definition should also change from inst to op.
2. Add the uop that calls a specializing function. In this case _SPECIALIZE_CONTAINS_OP.
3. The original CONTAINS_OP is now a new macro consisting of _SPECIALIZE_CONTAINS_OP + _CONTAINS_OP.
4. Define the cache structure in Include/internal/pycore_code.h. At the very least, a 16-bit counter is needed.
5. Write the specializing function itself in Python/specialize.c. Refer to any other function in that file for the format.
6. Remember to update operation stats by calling add_stat_dict in Python/specialize.c.
7. Add the cache layout in Lib/opcode.py so that Python's dis module will know how to represent it properly.
8. Bump magic number in Include/internal/pycore_magic_number.h.
9. Run make regen-all on *nix or build.bat --regen on PC.