Update empty state of Slack Application installation page when no projects are available

Fix the following comment from @lvanc:

I think for the "Without projects" case can be improve by adding some extra text/links around why and how to get projects in that list. This can be a follow up issue.

Users have to have at least maintainer level access to a project to be able to add Slack, so we can mention that condition here.

We can also include a link to the documentation for completeness.

Design source

✏ Figma project