more desktop user guide

Author: mbostock

docs/desktop-guide.html

@@ -50,7 +50,7 @@
 
     1. Open **Observable Desktop**, then click **Open browser**.
     1. If needed, **sign in** to your Observable account, or **create an account**.
-    1. Under **Authorize your device**, click **Continue**, then **Authorize**.
+    1. Click **Authorize the desktop app**.
     1. Close the **Success** window to return to Observable Desktop.
     1. Wait a few seconds for Observable Desktop to sign in.
   </script>
@@ -170,6 +170,42 @@
     3. Delete the cell with <span class="sans">**D**</span>.
     4. Clear the selection with <span class="sans">**esc ⎋**</span>.
   </script>
+  <script id="35" type="text/markdown">
+    ---
+
+    ## Cell modes
+
+    Cells can be written in JavaScript, Markdown, HTML, and several other languages. You can cycle between the primary modes when a cell is empty, or use the mode menu or edit menu to change the mode.
+  </script>
+  <script id="37" type="text/markdown">
+    <figure>
+      <img src="./videos/mode-empty.webp" loading="lazy">
+      <figcaption>Changing the mode of an empty cell</figcaption>
+    </figure>
+
+    1. Insert a new, empty cell.
+    2. Change the cell mode with <span class="sans">**down ↓**</span> or <span class="sans">**up ↑**</span>.
+  </script>
+  <script id="38" type="text/markdown">
+    <figure>
+      <img src="./videos/mode-keyboard.webp" loading="lazy">
+      <figcaption>Changing the mode via keyboard shortcuts</figcaption>
+    </figure>
+
+    1. Focus a cell, say by clicking on the editor.
+    2. Change the cell mode to JavaScript with <span class="sans">**command-1 ⌘1**</span>.
+    2. Change the cell mode to Markdown with <span class="sans">**command-2 ⌘2**</span>.
+    2. Change the cell mode to HTML with <span class="sans">**command-3 ⌘3**</span>.
+  </script>
+  <script id="39" type="text/markdown">
+    <figure>
+      <img src="./videos/mode-menu.webp" loading="lazy">
+      <figcaption>Changing the mode via the mode menu</figcaption>
+    </figure>
+
+    1. Focus a cell, say by clicking on the editor.
+    2. Change the cell mode using the **mode menu**.
+  </script>
   <script id="30" type="text/markdown">
     ---