Add help links and documentation for secrets manager UI

What does this MR do and why?

This adds help links and documentation for secrets manager UI. Since the secrets manager is currently an Experiment and a work in progress, we will not include this in the navigation yet.

I also did not include documentation for permissions and other features that are not yet fully available. They will be added in future frontend MRs as they are completed.

References

Resolves #470663 (closed)

Screenshots or screen recordings

In the designs, there's another Learn more link for the secrets table empty state. I think adding it here would be redundant since we're already adding one at the top of the page.

How to set up and validate locally

Setup

1. Add an Ultimate license to your gdk.
2. Set up openbao on your gdk.
3. Enable the Secrets Manager on gdk.
4. Make sure the Documentation pages URL setting is set to blank so the documentation is served through your gdk instead of redirecting to the production docs. You can set this from Admin > Settings > Preferences > Help page.

Verification

1. Go to your project settings (Settings > General) and expand Visibility, project features, permissions to view the permissions table. Verify that there is a new Learn more link.
2. Go to your the Secrets manager index page (Secure > Secrets manager) and expand Visibility, project features, permissions to view the permissions table. Verify that there is a new Learn more link.

MR acceptance checklist

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