BUG: created check and warning for merging dataframes on unequal inde…
…x levels (#13094)

- [ ] closes #13094
- [ ] tests added / passed
- [ ] warn users when merging between different index levels

Hello @SeeminSyed! Thanks for opening this PR. We checked the lines you've touched for [PEP 8](https://www.python.org/dev/peps/pep-0008) issues, and found:



* In the file [`pandas/tests/frame/test_axis_select_reindex.py`](https://github.com/pandas-dev/pandas/blob/7a9e0fede74e09ae8caaa41a00809c748a75c69f/pandas/tests/frame/test_axis_select_reindex.py):

> [Line 186:9](https://github.com/pandas-dev/pandas/blob/7a9e0fede74e09ae8caaa41a00809c748a75c69f/pandas/tests/frame/test_axis_select_reindex.py#L186): [E265](https://duckduckgo.com/?q=pep8%20E265) block comment should start with '# '
> [Line 191:13](https://github.com/pandas-dev/pandas/blob/7a9e0fede74e09ae8caaa41a00809c748a75c69f/pandas/tests/frame/test_axis_select_reindex.py#L191): [E128](https://duckduckgo.com/?q=pep8%20E128) continuation line under-indented for visual indent
> [Line 208:1](https://github.com/pandas-dev/pandas/blob/7a9e0fede74e09ae8caaa41a00809c748a75c69f/pandas/tests/frame/test_axis_select_reindex.py#L208): [W293](https://duckduckgo.com/?q=pep8%20W293) blank line contains whitespace

 Thanks @SeeminSyed closing as stale. ping to re-open.