Canvas limits what type of HTML a user can add to the Rich Content Editor, following mostly this page here: Canvas HTML Editor Allowlist - Instructure Community - 387066
Anything more than that and you'd need Admin rights and then add CSS or Javascript to the Canvas instance, or a third party solution that adds those to Canvas.
To achieve something like that within the HTML allow list, I believe the instructor would need to be willing to update the page every week with the current readings/assignments, and instead of a drop down, they can use the details and summary tags: HTML details Tag
Note that there's also an option to use tabs instead of details/summary, but that has been under a possible deprecation notice since at least 2017 (see the first comment): Making a Content Page with Tabs - Instructure Community - 279010
Alternatively, you can have a page with links for each week to either other pages or to modules with the readings and the assignments.