Add notifications to Slack App integration - App Home Update

Description

Now that we have an App Home in the GitLab Slack app, we can now communicate to users changes to our Slack App. Let's work on the content we'll share in the App Home for Adding notifications to the Slack app. A few things to consider:

• Features
• Benefits
• Uses
• Deprecations
• Migrations
• Any action required by the user
• Any opportunities to encourage adoption (e.g. try it now type features)

The most likely MVC here is:

Add a gif showcasing how the notifications work in the app, which aligns with our existing content with examples of slash commands.

Note: Since we updated the "About" tab in #381112 (closed), it would be redundant to mention Features, Benefits, Use.

Technical Proposal

We update Slack::BlockKit::AppHomeOpened to generate the new Block Kit JSON according to the designs.

We can put the changes to the App Home behind the same feature flag toggle as we use for #373321 (closed). This will mean when the flag is toggled on, the feature will go live in both GitLab.com and the App Home.

Note that changes to the App Home do not need a release of the Slack app.

Visual design proposal

See design management for mockups.

Design source

💻 Block kit