any reason for not using markdown specifically? 🤔

Using regular HTML tags comes in handy while specifying the dimensions of an image. Rest, nothing specific.

what are these for?

I was going by the common convention to put testing or development requirements in a separate file. The basic idea behind this was to install all developmental dependencies by invoking make commands in that respective directory.

Any reason you're using Makefile for this?

Facing issues while setting up a project is quite inevitable. To simplify the process, make commands might be invoked after specifying the tasks to be executed.
The sole purpose was to set the desired python version in the makefile. In my opinion, they make the Makefile cleaner and can be overridden by the user as per the future requirements.

Why create a separate requirements file and not use the project-level one?
This will likely get out of sync.

We can use the project-level requirements file for sure. No doubt that right now it does not serve any major purpose but as the size of the source code grows with the passage of time, specifying separate dependencies for production and development seems more reasonable. Moreover, in the case of working with unit testing frameworks, installing that in production would be unavailing.