DOC: Remove obsolete reference to CONTRIBUTING.md generation
It seems that a note was left over after https://github.com/pandas-dev/pandas/pull/45063 from the old CONTRIBUTING.md times ([still there in devdocs](https://pandas.pydata.org/docs/dev/development/contributing_documentation.html#about-the-pandas-documentation)). The note referred to generating the file from `contributing.rst` via pandoc, but that seems to no longer apply. Suspiciously these were also the only remaining hits for `git grep CONTRIBUTING`.

Removal seems straightforward enough, but I ran pre-commit just in case.
Thanks @adeak!