Enable `nitpicky = True` on doc configuration

[einarf]

This will validate all kinds of things such as label and docstring links. To make the docs maintainable this is more or less mandatory to enable. We want do know when links breaks and when having other undefined markup.

I spent a day removing the vast majority of them. The main blocker is that api docs are still not complete. We are missing things like module attributes. In addition the import resolver doesn't work for some parts of arcade such as the gui. That part is likely simple to resolve.

[einarf]

50% of the work is done here. There are blocking issues with autodoc resolving external references so nitpicky mode will always complain about those. There's also issues resolving module attributes. If we include them using e.g autodata they are not seen as existing members.

We have at least fixed most of the broken references now.