Edited Slack Docs

[maxehio]

Description of the change

At the bottom of our documentation for slack we say "Available Slash Commands", then there is little message that says:

It's recommended to instead use the notification policy in order to manage slack messages received from Spacelift. These slash commands are deprecated.

After that it says:

Three slash commands are currently available:
/spacelift subscribe $stackId - subscribes a particular Slack channel to run state changes for a given Stack - requires ;
/spacelift unsubscribe $stackId - unsubscribes a particular Slack channel from run state changes for a given Stack;
/spacelift trigger $stackId - triggers a tracked run for the specified Stack;

This is misleading if slack commands are deprecated so I just removed it altogether.

In addition, there is a paragraph that says:

Installing the Slack app doesn't automatically cause Spacelift to flood your Slack channels with torrents of notifications. These are set up on a per-stack basis using Slack commands and the management uses the Slack interface.

That hyperlink under "Slack commands" directs the user nowhere. So I adjusted that as well to direct a user to a helpful link.

Checklist

Please make sure that the proposed change checks all the boxes below before requesting a review:

• I have reviewed the guidelines for contributing to this repository.
• The preview looks fine.
• The tests pass.
• The commit history is clean and meaningful.
• The pull request is opened against the main branch.
• The pull request is no longer marked as a draft.
• You agree to license your contribution under the MIT license to Spacelift (not required for Spacelift employees).
• You have updated the navigation files correctly:
  • No new pages have been added, or;
  • Only nav.yaml has been updated because the changes only apply to SaaS, or;
  • Only nav.self-hosted.yaml has been updated because the changes only apply to Self-Hosted, or;
  • Both nav.yaml and nav.self-hosted.yaml have been updated.

[Daniellem97]

Notification policies are actually space wide, not per stack