Add nb::bytes::data() accessor

[nurpax]

Add nb::bytes::data() accessor as discussed in #569 (reply in thread)

Note: I couldn't get the tests to build on Windows due to this error:

C:\Users\janne\dev\nanobind\src\nb_internals.h(22,1): fatal error C1189: #error:  nanobind depends on tsl::robin_map, i
n particular version >= 1.3.0, <2.0.0 [C:\Users\janne\dev\nanobind\build\tests\nanobind-mydomain.vcxproj]
  Generating Code...

So trying if I can get test this with CI.

[wjakob]

Can you replace test_functions_ext.pyi.ref with your generated test_functions_ext.pyi? If you get the robin_map error message locally, you probably need to update your subrepository.

[nurpax]

Ahh, of course, that was it. All those years using git yet zero success in committing git submodules into CLI muscle memory.

If the change looks otherwise ok, I can add another commit for docs too.

[wjakob]

The change looks good, please go ahead.

[nurpax]

Added docs. Also updated the docs for nb::bytes(const void * buf, size_t n) constructor to match the actual code.

Hopefully the rst syntax is not too wrong in my change.

[wjakob]

Looks all good, thanks!