Draft: docs: quick start guide for beta customers

Relates to issue #583386

Changes

This MR introduces a comprehensive Beta quick start guide for GitLab Secrets Manager to help Beta customers get started quickly.

✅ Created Beta Quick Start Guide

File: doc/ci/secrets/secrets_manager/beta_quick_start.md

• Added YAML frontmatter with stage "Software Supply Chain Security", group "Pipeline Security"
• Included Hugo details shortcode for tier/offering (Ultimate, GitLab.com, GitLab Self-Managed, Beta status)
• Added Beta notice using Hugo alert shortcode explaining the closed Beta program
• Created comprehensive sections:
  • Overview explaining GitLab Secrets Manager and Beta program purpose
  • Prerequisites (Ultimate license, Owner role, admin configuration for self-managed)
  • Getting Started with step-by-step instructions for enabling and using Secrets Manager
  • Access Instructions with links to main documentation and reference to issue #550380
  • Demo Recording section with placeholder for PM to add video link
  • Known Limitations section with placeholder text for team to document limitations
  • Administrator Documentation with links to issues #573065 and #573278 (WIP)
  • Providing Feedback section explaining how Beta participants can provide feedback
  • Next Steps with links to related documentation

✅ Updated Main Secrets Manager Documentation

File: doc/ci/secrets/secrets_manager/_index.md

• Added link to the Beta quick start guide in the introduction section for easy discovery by Beta customers

✅ Updated External Secrets Index

File: doc/ci/secrets/_index.md

• Added GitLab Secrets Manager as the first item in the list of supported secret management providers with Beta designation

All documentation follows GitLab documentation style guide with proper Hugo shortcodes and maintains a welcoming tone for Beta participants.