Slack connector callback URL

[glebedel]

Add Slack connector guide with the correct OAuth callback URL and set up redirects for legacy paths.

The Slack guide was not present in the repository, so it was created to include the specified callback URL and ensure existing documentation links remain functional.

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Slack Thread

 

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Summary by cubic

Added a Slack connection guide with the correct OAuth callback URL and updated redirects so old links point to the new guide. This makes Slack OAuth setup clear and keeps existing documentation links working.

• New Features

  • New guide at /connection-guides/messaging/slack with OAuth setup steps and redirect URL: https://api.stackone.com/connect/oauth2/slack/callback.
  • Added Slack tile under the Messaging category on the Integrations page.

• Migration

  • Redirects added from /integration-guides/messaging/slack (and trailing slash) to the new path; no app changes needed.
  • Update any docs or bookmarks to /connection-guides/messaging/slack.

^{Written for commit 9a2887e. Summary will update automatically on new commits.}

[cursor]

Cursor Agent can help with this pull request. Just @cursor in comments and I'll start working on changes in this branch.
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