DOC: Add tips on where to locate a test

[rhshadrach]

Pandas version checks

• I have checked that the issue still exists on the latest versions of the docs on main here

Location of the documentation

https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#writing-tests

Documentation problem

NA

Suggested fix for documentation

As a general tip, we should mention searching for the methods you want to add a test for using either grep or IDE search functionality. Also, I think it would be helpful to state that there is at times no clear location to put a test, take your best guess, and you may be requested by reviewers to move it.

[georgemunyoro]

take