fix(site): exclude workspace schedule settings for prebuilt workspaces (#18826)

## Description

This PR updates the UI to avoid rendering workspace schedule settings
(autostop, autostart, etc.) for prebuilt workspaces. Instead, it
displays an informational message with a link to the relevant
documentation.

## Changes

* Introduce `IsPrebuild` parameter to `convertWorkspace` to indicate
whether the workspace is a prebuild.
* Prevent the Workspace Schedule settings form from rendering in the UI
for prebuilt workspaces.
* Display an info alert with a link to documentation when viewing a
prebuilt workspace.

<img width="2980" height="864" alt="Screenshot 2025-07-10 at 13 16 13"
src="https://github.com/user-attachments/assets/5f831c21-50bb-4e05-beea-dbeb930ddff8"
/>


Relates with: https://github.com/coder/coder/pull/18762

---------

Co-authored-by: BrunoQuaresma <bruno_nonato_quaresma@hotmail.com>

coderd/workspaces.go

@@ -2231,6 +2231,7 @@ func convertWorkspace(
 	if latestAppStatus.ID == uuid.Nil {
 		appStatus = nil
 	}
+
 	return codersdk.Workspace{
 		ID:                                   workspace.ID,
 		CreatedAt:                            workspace.CreatedAt,
@@ -2265,6 +2266,7 @@ func convertWorkspace(
 		AllowRenames:     allowRenames,
 		Favorite:         requesterFavorite,
 		NextStartAt:      nextStartAt,
+		IsPrebuild:       workspace.IsPrebuild(),
 	}, nil
 }