How to find the Content Library
You can access your saved items under the Templates tab and by selecting 'Content Library' from the lower horizontal menu.
Saving content to your Content Library
To save content just open any proposal or template in edit mode. From there you're looking for the save icon. You'll find it in 2 different places depending on whether it's a content block or a page.
What content can you add to the Content Library
Full width images
Pages (this will save any content blocks, videos and full width images on the page)
Saving Content blocks, Videos and Full-width images
Just hover over the content block, image or video you want to save and click the disk icon. This will bring up the sidebar, enter the name you'd like to save this content as and click "Save" and a confirmation window will popup confirming your element has been saved to the content library. Click 'OK' and that's it.
Saving sections can be done hovering over the section you want to save and click the little settings cog to bring up the sidebar, then click 'Save this section to the Content Library'
Check out this article to learn how to add items from your Content Library into any Proposal or Template.