[Docs] Remove all references to the "Workspace Manager" Role

[Copilot]

Notes for Reviewers

This PR fixes #1663

The "Workspace Manager" role was never fully realized in Cloud. Removes all documentation references to this role.

Changes:

• content/en/cloud/security/roles/workspace-roles.md - Updated description to reflect single role availability
• content/en/cloud/security/roles/_index.md - Removed Workspace Manager card section and updated image alt text

Signed commits

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Original prompt

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This section details on the original issue you should resolve

<issue_title>[Docs] Remove all references to the "Workspace Manager" Role</issue_title>
<issue_description>#### Current State
The "Workspace Manager" Role was never fully realized in Cloud.

Desired State

Neither Cloud nor Kanvas documentation should refer to such a role.

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  </issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes [Docs] Remove all references to the "Workspace Manager" Role #856

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