Add 19.5 release note for GitLab Orbit GA

What does this MR do?

Adds the GitLab Orbit GA feature release note to the GitLab 19.5 release notes. Orbit is an out-of-cycle launch on Oct 29, 2026, after 19.5 shipped on Oct 15. This MR follows the Update content after a release process.

Created by Claude on behalf of @thelle.

Release note

• File: doc/releases/19/gitlab-19-5-released/gitlab-orbit-ga.md
• Title: GitLab Orbit is generally available
• Tier: Premium, Ultimate
• Offering: GitLab.com only.
• Stage: knowledge_graph, which groups under Unified DevOps and Security
• Category: Orbit, from categories.yml
• Placement: level: primary and weight: 10, so it appears in Primary features

Review checklist

• PM review: @michaelangeloio
• Engineering Manager review
• Technical Writer review of the metadata and body text
• Confirm the documentation_link (../../../orbit/) resolves. Orbit docs are published from the knowledge-graph project, not from this repository.
• Confirm the work item (gitlab-org&23874) isn't confidential-replacing with #633557
• After merge, backport to 19-5-stable-ee and run a docs-gitlab-com pipeline for 19.5 (Technical Writer)

Timing

Merge on Oct 29, 2026, with the 5:30 AM PT web updates, after GitLab.com availability is confirmed. The What's New page header links to these release notes.

Related to gitlab-com/marketing/digital-experience/about-gitlab-com#2250