feat: include `XML` files while applying a `XSL` transformation

[studerluk]

This feature streamlines the conversion of Doxygen XML output into documents while giving the oportunity to structure the resulting document as needed. It gives a bit more control than for example Sphinx/Breathe

This includes:

• Additional dependency to lxml
• Adding xsl option to include line

[DCsunset]

Hi, do you have a minimal example to showcase the new feature (as well as testing it)? I'm not familiar with Doxygen XML so it's hard for me to test it.

[studerluk]

I added a simple test using this feature.

I also generally renamed everything from xsl to xslt. I think this results in a more consitent naming.

[DCsunset]

Thanks. It looks good to me.