gazebo-setup: clarify location setup before adding a machine

[ya8282]

Summary

In Step 2: Create a Machine in Viam, clicking + Add machine triggers a dialog asking the user to select a location first. The current instructions don't account for users who have no locations set up, or who have multiple locations and need to choose one.

Adds a dedicated step with two labeled conditions:

• No locations yet: directs the user to create one via + Add location
• Multiple locations: directs the user to select the right one before proceeding

Before:

1. Go to app.viam.com and log in
2. Click the **Locations** tab
3. Click **+ Add machine**
4. Name it `inspection-station-1`
5. Click **Add machine**

After:

1. Go to app.viam.com and log in
2. Click the **Locations** tab
3. Select a location:
   - **No locations yet:** Click **+ Add location**, enter a name (for example, `my-location`), and click **Add**
   - **Multiple locations:** Select the location you want to use
4. Click **+ Add machine**
5. Name it `inspection-station-1`
6. Click **Add machine**

🤖 Generated with Claude Code

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