Some more tests testing rendering functionality

Again mostly LLM generated but manually checked and corrected.

Overhaul chapter/header counting in renderer

Using static variables makes it impossible to automatically test this
correctly.

The PDF renderer is now a singleton. The Writer class initializes it
an

Overhaul chapter/header counting in renderer

Using static variables makes it impossible to automatically test this
correctly.

The PDF renderer is now a singleton. The Writer class initializes it
anew for each export, but during the export, the same renderer is
reused.

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introduce a first test for rendering - testing numbered headlines

The test passes, but I am not 100% sure the behaviour is correct yet.
Static variables in the renderer might be a problem. Not teste

introduce a first test for rendering - testing numbered headlines

The test passes, but I am not 100% sure the behaviour is correct yet.
Static variables in the renderer might be a problem. Not tested with
multiple documents yet.

Currently render options need to be set via global conf and are not
passed from the Config object.

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