Remove broken link to NumPy's `HOWTO_DOCUMENT.rst.txt`
- [ ] closes #xxxx (Replace xxxx with the GitHub issue number)
- [x] [Tests added and passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#writing-tests) if fixing a bug or adding a new feature
- [x] All [code checks passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit).
- [x] Added [type annotations](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#type-hints) to new arguments/methods/functions.
- [ ] Added an entry in the latest `doc/source/whatsnew/vX.X.X.rst` file if fixing a bug or adding a new feature.

This link is broken (for 4+ years). The "fixed" link is https://github.com/numpy/numpy/blob/main/doc/HOWTO_DOCUMENT.rst which simply points to the same numpydoc page that pandas does here.

The "original" document I think is here: 
https://github.com/numpy/numpy/blob/254c50af3a52c8b1444e46857e9ec59fcb212a41/doc/HOWTO_DOCUMENT.rst.txt

which may have evolved to this:
https://numpy.org/doc/stable/dev/howto-docs.html

I think it's easiest/clearest to simply delete this link, but I can update the link or keep a comment in parentheses if desired.
thx @eriknw 