feat: tutorial configuring slack notifications on run failure

[Ben-Epstein]

As per slack conversation with @cicdw

walkthrough configuring slack notifications in code for prefect run failures

Checklist

- [ ] This pull request references any related issue by including "closes <link to issue>"
- If no issue exists and your change is not a small fix, please create an issue first.
- [ ] If this pull request adds new functionality, it includes unit tests that cover the changes
- [ ] If this pull request removes docs files, it includes redirect settings in mint.json.
- [ ] If this pull request adds functions or classes, it includes helpful docstrings.

[Ben-Epstein]

not totally sure how to handle the linting error

[gw0] linux -- Python 3.9.21 /home/runner/work/prefect/prefect/.venv/bin/python3
Error in code block:
     ```
93   from prefect import flow
94   from notify import notify_slack
95   
96   
97   @flow(on_failure=[notify_slack], on_crashed=[notify_slack], on_cancellation=[notify_slack])
98   async def main():
99       ...
     ```
Traceback (most recent call last):
  File "/home/runner/work/prefect/prefect/docs/v3/tutorials/slack.mdx", line 94, in <module>
    from notify import notify_slack
ModuleNotFoundError: No module named 'notify'

since this is a documentation example. We could add a type: ignore or noqa but that seems weird in a documentation guide