Docs: Add GitLab Secrets Manager to user docs

• Start this issue's title with Docs: or Docs feedback:.

Problem to solve

We should update our documentation to introduce the Secrets Manager to our users, and help them differentiate it from other features we currently provide. The docs should help users make the decision on what is the best service to use for their specific use case.

Further details

Proposal

We have a general Pipeline Security entry page in the docs. For this issue, we should cover:

• What is the Secrets Manager? How does GitLab integrate with Openbao?
• The differences between using CI/CD Variables versus Secrets Manager
• The differences between existing third-party integrations (External Secrets) versus using GitLab's native Secrets Manager

How to use secrets created with the Secrets Manager is covered by Docs: Basic documentation for Secrets Manager, ... (#470663 - closed).

Who can address the issue

This is a collaboration between grouppipeline security PM, engineers involved in the implementation, and tech writer for the group.

Other links/references

Documentation Links:

• Pipeline Security entry page
• CI/CD Variables
• External Secrets (Vault by HashiCorp, Google Cloud Secret Manager, Azure Key Vault)
• CI/CD YAML syntax reference

/cc @jocelynjane @mgandres