sogo: highlight tb minimal setup

calendar/thunderbird_tbsync_caldav.markdown

 # Calendar sync with Thunderbird
 
+## Minimal setup
+
 SOGo and Thunderbird can sync out of the box using the CalDAV protocol. In order to set it up, just right click in the calendar tab of Thunderbird, choose `New calendar`, `On the Network`, and then provide your D-PHYS login and `https://sogo.phys.ethz.ch/SOGo/dav` for `Location`. Thunderbird will then offer you all calendars you have access to.
 
+## Alternative setup with addons
+
 If you find yourself adding/removing calendars frequently, you can use the TbSync plugin for Thunderbird that offers more flexibility in calendar management.
 
-## Install required addons
+### Install required addons
 
 Install [TbSync](https://addons.thunderbird.net/en-US/thunderbird/addon/tbsync/):
 
@@ -18,8 +22,7 @@ Install [Provider for CalDAV & CardDAV](https://addons.thunderbird.net/en-US/thu
 - Find more add-ons: `caldav` > **Provider for CalDAV & CardDAV** > **Add to Thunderbird**
 - Confirm any following popups with **Add** or **OK**
 
-
-## Configure TbSync
+### Configure TbSync
 
 - Select **Tools** > **Synchronization Settings (TbSync)**
 - In the **Account settings** > **Account actions** drop-down menu, select **Add new account** > **CalDAV & CardDAV**