DOC add examples to Timestamp.unit
xref https://github.com/pandas-dev/pandas/issues/37875

Just an example of how to open such a PR

Before submitting, you should run
```
python scripts/validate_docstrings.py pandas.Timestamp.unit
```
and check that `No examples section found` doesn't appear in the "errors found" section (if other errors appear, it's probably fine for now)

You should then also remove the function you've fixed up to the exclusions in `ci/code_checks.sh` under the EX01 section

Thanks @MarcoGorelli 