Nanobind support in the python doc generator

[mosra]

Just a rough initial sketch for #261. Quite some things can be reused from pybind11, especially the signature parsing. Other things such as detecting what's an enum or or what's a function have to be redone for nanobind.

Things to do:

• Make the nanobind tests opt-in so we can still test pybind11 w/ older cmake
• Nothing should rely on PYBIND11_COMPATIBILITY being enabled
• Aim for as much test reuse with pybind11 as possible, if the output can be 1:1 it'd be great
  • Unnamed arguments seem to be named differently (arg vs arg0?), that may also affect static/class/method distinction
    • I think I had a stash fixing this for pybind in a better way? Adding real support for classmethods and such. Fix that first
• Port all tests, not just a subset of one
• Anything else to do for pybind first, and then just port to nanobind? Enum value docs etc.

Right now I don't think I can find time to finish this, wanted mainly to see how hard it would be -- it seems a manageable scope.