Refactor docs for slash commands

What does this MR do and why?

• Refactors the following pages/topics for slash commands:
  • Slash commands in Mattermost and Slack
  • GitLab for Slack app: Slash commands
  • Mattermost slash commands: Available slash commands
  • Slack slash commands: Slash commands
• Ensures a single source of truth in GitLab for Slack app: Slash commands
• Redirects Slash commands in Mattermost and Slack to GitLab for Slack app: Slash commands

Follow-up

• Delete the nav entry for Slash commands in Mattermost and Slack

Screenshots or screen recordings

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Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.