We actually do have a doc for adding in a button like that: https://docs.shopify.com/manual/configuration/store-customization/page-specific/cart-page/add-a-back...
Here's how you can set that up.
1. Head to Online Store > Themes > Customize Theme, then click Edit HTML and CSS. This takes you to all the coding pages for your store.
2. In this tutorial, we're looking for the Cart code, so open up cart.liquid to check that out. You have the option to send customers either to the main Catalog (all of your products) or the most recent Collection they viewed. Choose whichever code you prefer.
3. To show it before the checkout button, paste it to line 74. To paste it after the checkout button, paste it to line 77.
If you need any help, just give me a shout!