That page (well, with this corrected URL: https://developer.atlassian.com/platform/forge/custom-ui/) is part of Atlassian's documentation for developers of applications for Confluence Cloud using Forge.
Here's details on that: https://developer.atlassian.com/platform/forge/
And more generally about application development: https://developer.atlassian.com/developer-guide/cloud-development-platform-overview/
So... as an end-user (or administrator), this won't help you create buttons or drop-downs.
Hope this clears things up.
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