replace Appender decorator with doc
- [x] ref #31942
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] replaces Appender with doc for NDFrame.compare()

Don't you need to Series.compare and DataFrame.compare too? > Don't you need to Series.compare and DataFrame.compare too?

Yes. Done.

I kept the use to of `_shared_docs["compare"]` to keep the PR small but I'd find it more intuitive to define the doc string on `NDFrame.compare()` and then refer to it in the calls to the doc decorator on `Series.compare()` and `DataFrame.compare()`.
 > Yes. Done.

Have you checked locally that you get the correctly-rendered docstrings?  When I try it with this PR, pandas cant even be imported. > > Yes. Done.
> 
> Have you checked locally that you get the correctly-rendered docstrings? When I try it with this PR, pandas cant even be imported.

You were right. The CI is passing now. thanks @smartvinnetou 