docs: add code monitoring slack tutorial

Created by: limitedmage

Fixes #27161 (closed)

Also fixes some links in the docs related to code monitoring

The markdown files have videos but the GitHub PR preview doesn't show them. Here are the videos:

https://storage.googleapis.com/sourcegraph-assets/search/code-monitoring/slack-tutorial/1-create-app.mp4 https://storage.googleapis.com/sourcegraph-assets/search/code-monitoring/slack-tutorial/2-create-webhook.mp4 https://storage.googleapis.com/sourcegraph-assets/search/code-monitoring/slack-tutorial/3-create-monitor.mp4