help: Add mobile instructions to "Share and upload files".

- Documents uploading files via compose box buttons.
- Documents sharing files from other Android apps.

static/images/help/mobile-camera-icon.svg

@@ -0,0 +1,4 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 24 24" fill="none" stroke="#6492fd" stroke-width="2" stroke-linecap="round" stroke-linejoin="round" class="feather feather-camera">
+    <path d="M23 19a2 2 0 0 1-2 2H3a2 2 0 0 1-2-2V8a2 2 0 0 1 2-2h4l2-3h6l2 3h4a2 2 0 0 1 2 2z"></path>
+    <circle cx="12" cy="13" r="4"></circle>
+</svg>

static/images/help/mobile-image-icon.svg

@@ -0,0 +1,5 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 24 24" fill="none" stroke="#6492fd" stroke-width="2" stroke-linecap="round" stroke-linejoin="round" class="feather feather-image">
+    <rect x="3" y="3" width="18" height="18" rx="2" ry="2"></rect>
+    <circle cx="8.5" cy="8.5" r="1.5"></circle>
+    <polyline points="21 15 16 10 5 21"></polyline>
+</svg>

static/images/help/mobile-paperclip-icon.svg

@@ -0,0 +1,3 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 24 24" fill="none" stroke="#6492fd" stroke-width="2" stroke-linecap="round" stroke-linejoin="round" class="feather feather-paperclip">
+    <path d="M21.44 11.05l-9.19 9.19a6 6 0 0 1-8.49-8.49l9.19-9.19a4 4 0 0 1 5.66 5.66l-9.2 9.19a2 2 0 0 1-2.83-2.83l8.49-8.48"></path>
+</svg>