Update guidance on Git (#211)

Updating the '[Setting up
Git](https://prototype-kit.service-manual.nhs.uk/how-tos/git)' guidance
with 'how to check if you already have Git installed'.

(Issue
#209)

| Before | After |
| -------|-------|
|
![image](https://github.com/user-attachments/assets/cad0ad6e-81c2-4aee-a8e5-2b03b4ce404a)
|
![image](https://github.com/user-attachments/assets/6140f5e7-9d21-4b4d-990b-77a774552c21)
|

app/views/how-tos/git.html

@@ -25,9 +25,21 @@ <h1>
         <p>Git is not the same as GitHub. Git stores versions of your work, and lets you collaborate more easily with others. GitHub puts it all online with a web interface.</p>
       </div>
 
-      <h2 id="install-git">1. Install Git</h2>
+      <h2 id="install-git">1. Check if you have Git installed</h2>
 
-      <p>The first thing you need to do is <a href="https://git-scm.com/downloads" target="_blank" rel="noopener">install Git (opens in new tab)</a>.</p>
+      <p>Open a terminal, type <code class="app-code">git --version</code> and press enter.</p>
+
+      <p>If git is installed you will see something similar to this:</p>
+
+      <pre class="app-pre"><code class="app-code"></code>git version 1.9.1</code></pre>
+
+      <p>You can now <a href="#git-user-password">set a username and password</a>.</p>
+
+      <p>If git is not installed you will see this:</p>
+
+      <pre class="app-pre"><code class="app-code"></code>$ git: command not found</code></pre>
+
+      <p>You will need to <a href="https://git-scm.com/downloads" target="_blank" rel="noopener">install Git (opens in new tab)</a> before continuing.</p>
 
       <h2 id="git-user-password">2. Set a username and password</h2>