How can I add links to the Left Navigation in your Moodle site?
Unfortunately, Left Navigation/Sidebar is not configurable and you cannot add links/options/menu to it.
You can make use of this plugin to add a single level link in the left navigation: Local navigation
You can also add those options to the header menu. Please refer to this documentation to know more about this - https://docs.moodle.org/36/en/Theme_settings#Custom_menu_items
Please note that the left navigation is no longer available in Moodle 4.0 and above