DOC: Add missing doc `Index.array` and `Index.to_numpy`
- [ ] closes #xxxx (Replace xxxx with the GitHub issue number)
- [ ] [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
- [ ] All [code checks passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit).
- [ ] 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.

As https://github.com/pandas-dev/pandas/pull/57029#issuecomment-1935456363 and #57016, it appears that we are lacking some API references.

I'm interested in conducting a thorough check for any additional missing documentation, but haven't been able to identify a consistent pattern in the missing documentation. Do we have any other suggestions other than comparing the different versions one by one?
thx @luke396 