Add custom buttons and description of how to add them to mobile frontend.

Custom-buttons.md

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+You can create custom buttons that run your frontend code, and assign shortcuts to them. The default database has an example of a button that opens today's page:
+
+```javascript
+api.addButtonToToolbar({
+    title: 'Today',
+    icon: 'calendar',
+    shortcut: 'alt+t',
+    action: async function() {
+        const todayNote = await api.getTodayNote();
+
+        await api.waitUntilSynced();
+        
+        api.activateNote(todayNote.noteId);
+    }
+});
+```
+
+You can add the button to Trilium by creating a note of type "JS frontend" with this code.
+
+To add the button on startup of the "desktop frontend", add a `#run=frontendStartup` attribute.
+
+To add the button into the "mobile frontend", add `#run=mobileStartup`.