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>

codersdk/workspaces.go

@@ -66,6 +66,12 @@ type Workspace struct {
 	AllowRenames     bool             `json:"allow_renames"`
 	Favorite         bool             `json:"favorite"`
 	NextStartAt      *time.Time       `json:"next_start_at" format:"date-time"`
+	// IsPrebuild indicates whether the workspace is a prebuilt workspace.
+	// Prebuilt workspaces are owned by the prebuilds system user and have specific behavior,
+	// such as being managed differently from regular workspaces.
+	// Once a prebuilt workspace is claimed by a user, it transitions to a regular workspace,
+	// and IsPrebuild returns false.
+	IsPrebuild bool `json:"is_prebuild"`
 }
 
 func (w Workspace) FullName() string {