Fix (Analytics): Fix custom dashboard description

[TuannasCodeCatastrophe]

Description

Definition of done
Adjust the "Dashboard permissions only control access to the dashboard object. Access to the underlying data sources (such as a control plane) is enforced separately." description on this page with this feedback: "Maybe we can elaborate here a bit on "Access to the underlying data sources ..." to improve the AI response. Basically, dashboard permissions (viewer role today) only control access to the dashboard object; underlying data access still needs to be configured by who ever creates the dashboards using preset filters. Like if you want a user who only has access to Control Plane A, you need to add a preset filter for Control Plane A. Otherwise, without the filter, a user may be able to open a dashboard but see empty or partial results if they lack permissions. I'd also remove sources "data sources" and just keep "data" around."
Information
DRP: Christian H.

Size
Small

Fixes #3089

Preview Links

https://deploy-preview-3839--kongdeveloper.netlify.app/advanced-analytics/custom-dashboards-reference/#permissions

Checklist

• Tested how-to docs. If not, note why here.
• All pages contain metadata.
• Any new docs link to existing docs.
• All autogenerated instructions render correctly (API, decK, Konnect, Kong Manager).
• Style guide (capitalized gateway entities, placeholder URLs) implemented correctly.
• Every page has a description entry in frontmatter.
• Add new pages to the product documentation index (if applicable).

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