ENH: add HPyContextVar* interfaces and document how it is done

[mattip]

I added the missing HPyContextVars_* API functions.

• I documented what I did in docs/api-reference/index.rst
• I changed the following files then ran `make autogen, which failed:
  • hpy/tools/autogen/public_api.h
  • hpy/tools/autogen/conf.py
• I added HPyContextVar_Get to NO_WRAPPER in hpy/tools/autogen/debug.py, and then needed to modify/add these files:
  • hyp/devel/src/runtime/ctx_contextvars.c (and added that to setup.py)
  • hpy/debug/src/debug_ctx.c
  • hpy/devel/include/hpy/runtime/ctx_funcs.h
  • (edit) hpy/devel/include/hpy/cpython/misc.h
• I added a test. It passes. universal and debug mode, but fails to compile on cpython mode: it is missing the declaration for HPyContextVar_Get. Where do I need to add it?

[fangerer]

Thanks, @mattip, for this PR. I added some comments.
Most of my comments are not strict except of the once where we need to write to the output var even in case of errors.