I've been working on some edits for my own site (ePalmetto Global) and thought a quick tutorial on how to customize the sidebar in Google Site might be helpful.
How to Customize The Sidebar in the Bootstrap Template
It's quite simple to edit the contents of your sidebar and there is only one real limitation with it. You can't add google gadgets to the sidebar. It is the only place where gadgets can't be added.
Add or remove items from your sidebar
1. Site owners can add and remove items in the site sidebar by clicking the "Edit sidebar" link at the bottom of the sidebar.
2. You can edit, delete, or add new items to the sidebar. Click "Add a sidebar item".
3. In the dialog, select the type of page element you want to add.
4. The item is now added. If you want to organize the elements, you can reorder by dragging the boxes.
1. Click "Change site layout".
2. In the dialog you can select the location of the sidebar and the width. When you are done, click "OK".
3. Click "Save changes".
4. Once your changes have been confirmed, click "Return to site".