Add showAdminBar docs

[BoDonkey]

Summary

Summarize the changes briefly, including which issue/ticket this resolves. If it closes an existing Github issue, include "Closes #[issue number]" This PR documents the new getShowAdminBar() method and closes PRO-6086.

What are the specific steps to test this change?

For example:

1. Run the website and log in as an admin
2. Open a piece manager modal and select several pieces
3. Click the "Archive" button on the top left of the manager and confirm that it should proceed
4. Check that all pieces have been archived properly

1. Spin up the docs site
2. Navigate to http://localhost:5173/guide/custom-ui.html#toggling-the-admin-bar-visibility and http://localhost:5173/reference/modules/admin-bar.html#getshowadminbar-req

   What kind of change does this PR introduce?

   (Check at least one)

• [ ] Bug fix
• [ ] New feature
• [ ] Refactor
• [X] Documentation
• [ ] Build-related changes
• [ ] Other

Make sure the PR fulfills these requirements:

• [X] It includes a) the existing issue ID being resolved, b) a convincing reason for adding this feature, or c) a clear description of the bug it resolves
• [ ] The changelog is updated
• [ ] Related documentation has been updated
• [ ] Related tests have been updated

If adding a new feature without an already open issue, it's best to open a feature request issue first and wait for approval before working on it.

Other information:

[linear[bot]]

PRO-6086 As a developer, I can learn how to suppress the admin bar for guests etc.