[#29801] Add "Slack Slash Commands" to API docs

What does this MR do?

Add documentation for the Slack Slash Command

Why was this MR needed?

Not known that Slack Slash Command is supported from the API https://gitlab.com/gitlab-org/gitlab-ce/issues/29801

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Squashed related commits together

What are the relevant issue numbers?

Closes #29801 (closed)