Use "GitLab Secrets Manager" in documentation, UI, and error messages

Why are we doing this work

Based on this decision:

The team decided to use capitalized "GitLab Secrets Manager" when referring to it as a proper noun. The decision includes these key points:

1. When referring specifically to GitLab's native secrets manager feature, it should be written as "GitLab Secrets Manager" (capitalized)
2. When discussing secrets management solutions in general, lowercase "secrets manager" should be used

This decision was influenced by:

• Comparison with competitor naming conventions (AWS Secrets Manager, Google Cloud Secret Manager)
• Similarity to other standalone GitLab services like GitLab Runner and GitLab Pages
• Consistency when mentioned alongside other provider services

The capitalization standard will be applied consistently across documentation, UI, and error messages.

This issue is about applying the naming convention across documentation, UI, and error messages.

Additionally, based on !206090 (comment 2773638773), let's also replace the use of generic Secrets namespace with more granular subcategory namespaces.

Relevant links

See Discussion: Clarify capitalization for Secrets ... (#558520 - closed) for the discussion on naming.

Implementation plan

• Update documentation
• Update UI text
• Update error messages
• Replace Secrets namespace with more granular subcategory namespaces

Verification steps

Check that the docs and UI are updated with the new text. To setup the secrets manager on gdk:

Setup

1. Make sure your gdk is on a Premium+ license.
2. Set up openbao on your gdk.
3. Enable the secrets manager, then go to /path-to-project/-/secrets