Add docs for slack integration

[grace-rehn]

Add docs for #project-od-slack-integration

• Adds a new docs page for the new slack integration with details on how to configure it using the wizard
• Adds a new docs page for the new Send a Slack Message built in step
• Updates the existing Subscription docs with a slack notification section
• Updates the existing Notification docs to mention new Send a Slack Message built in step

Fixes HPY-1503

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[steve-fenton-octopus]

LGTM. Thanks for fixing the markdown lint errors along the way, too!

[michelle-luana]

The content itself is great. My only thought was whether we want to add some screenshots to break up the text / add context?

[grace-rehn]

My only thought was whether we want to add some screenshots to break up the text / add context?

@michelle-luana yeah sounds good, I can add one on the slack integration page of the wizard and one on the slack step page

[grace-rehn]

Screenshots added: