docs: add OPA playground query example

[omribz156]

Fixes Issue

Closes #242

Changes proposed

• Add an OPA playground UI query example to the OPAL Playground quickstart.
• Show the query for finding users with the admin role.
• Link the expected result back to the example policy repo data.json so readers can see where alice comes from.

Check List (Check all the applicable boxes)

• I sign off on contributing this submission to open-source
• My code follows the code style of this project.
• My change requires changes to the documentation.
• I have updated the documentation accordingly.
• All new and existing tests passed.
• This PR does not contain plagiarized content.
• The title of my pull request is a short description of the requested changes.

Screenshots

Not applicable; docs text only.

Note to reviewers

Verification:

• Checked the current permitio/opal-example-policy-repo data.json and rbac.rego via GitHub API before adding the query/result.
• yarn install --frozen-lockfile in documentation/
• yarn build in documentation/

The docs build passed. It still reports existing broken anchors on unrelated pages, including #compose-recap, #generate-secret, and several tutorial anchors.

This was implemented with Codex assistance and manually reviewed before opening the PR.

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