SuiteCRM structure has changed abit from 7 and 8 but even more so from Suga. Don’t rely on SugarCRM documentation regarding to front end extension especially if it’s Sugar 7+
The aspect of adding buttons to the view defs hasn’t changed too much from 7 to 8. Lets review what you’ve got.
If you don’t have a file in legacy/custom/modules/Accounts/metadata then you can copy one that exists already in the modules folder: legacy/modules/Accounts/metadata/detailviewdefs
In that file (copied to the custom folder if not already there) you need to add a button to the templateMeta => form => buttons section of the array.
There you can add an addition button.
This post has some good tutorials to help, you may need to change the pathnames since you’ll be looking into legacy/ etc