Restore Slack notifications documentation page (19.2 backport)

What does this MR do and why?

Backport of !253599 (merged) to 19.2.

The Slack notifications page carried removal markings with a remove_date of 2026-05-16. After that date passed, the content was deleted as routine docs maintenance in !240909 (merged), and the page was reduced to a redirect stub.

Removal of the integration itself has since been called off, because there is no clear migration path yet. The integration is still available and still deprecated, with no removal date set, and customers are still advised to move to the GitLab for Slack app.

GitLab 19.2 and 19.3 both shipped with the page gutted, so 19.1 is the last published version that has the full page. This restores it.

Because 19.2 is older than the current stable release, this backport needs Technical Writing leadership approval and a release manager to merge it. Approval is being tracked in gitlab-org/technical-writing/team-tasks#1729 (closed). This MR stays a draft until that approval is in.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• This MR is backporting a bug fix, documentation update, or spec fix, previously merged in the default branch.
• The MR that fixed the bug on the default branch has been deployed to GitLab.com (not applicable for documentation or spec changes).
• The MR title is descriptive (e.g. "Backport of 'title of default branch MR'"). This is important, since the title will be copied to the patch blog post.
• Required labels have been applied to this merge request
  • severity label and bug subtype labels (if applicable)
  • If this MR fixes a bug that affects customers, the customer label has been applied.
• This MR has been approved by a maintainer (only one approval is required).
• Ensure the e2e:test-on-omnibus-ee job has succeeded, or if it has failed, investigate the failures. If you determine the failures are unrelated, you may proceed. If you need assistance investigating, request help in the #s_developer_experience Slack channel to confirm the failures are unrelated to the merge request.

Note to the merge request author and maintainer

If you have questions about the patch release process, please:

• Refer to the patch release runbook for engineers and maintainers for guidance.
• Ask questions on the #releases Slack channel (internal only).
• Once the backport has been merged, the commit changes will be automatically deployed to a release environment that can be used for manual validation. See after merging runbook for details.