Fix links to missing CPython files

This is a follow-up of #984:

There are a few broken links that I haven't touched yet:

(developer-workflow/grammar: line 40) broken https://github.com/python/cpython/blob/main/Include/Python-ast.h
(developer-workflow/grammar: line 33) broken https://github.com/python/cpython/blob/main/Include/token.h
(internals/compiler: line 516) broken https://github.com/python/cpython/blob/main/Include/code.h
(internals/compiler: line 488) broken https://github.com/python/cpython/blob/main/Python/peephole.c

• Include/Python-ast.h was removed by @vstinner
  • https://github.com/python/cpython/pull/24933
• Include/token.h was removed by @vstinner
  • https://github.com/python/cpython/pull/92652
• Include/code.h was removed by @vstinner
  • https://github.com/python/cpython/pull/32385
• Python/peephole.c was removed by @markshannon
  • https://github.com/python/cpython/pull/21517

The respective sections in developer-workflow/grammar.rst and internals/compiler.rst might need to be revisited.
@vstinner and @markshannon: can you advise on whether these are quick fixes that we can include in this PR or if they should be handled separately?

I found another file mentioned in the devguide that was deleted:

• Python/wordcode_helpers.h was removed by @brandtbucher
  • https://github.com/python/cpython/pull/31543