api: make buffer public api

[jhamman]

This PR moves zarr.core.buffer to zarr.abc.buffer and zarr.buffer, paving the way to make the rest of zarr.core private in a follow up PR.

Towards #2621

[jhamman]

put this in to prompt a discussion -- numcodecs depends on this module already... which sucks.

@normanrz - do you have thoughts on how to proceed? Options are:

1. keep zarr.core but document it as private. We've managed to do this in the Xarray project with few issues.
2. change the import in numcodecs and make a new release

[dstansby]

We can easily skip documenting some 'public' (without a _) modules, which would probably help:

zarr-python/docs/conf.py

Lines 64 to 76 in f9c2024

	def skip_submodules(
	app: sphinx.application.Sphinx,
	what: str,
	name: str,
	obj: object,
	skip: bool,
	options: dict[str, Any]
	) -> bool:
	# Skip documenting zarr.codecs submodules
	# codecs are documented in the main zarr.codecs namespace
	if what == "module" and name.startswith("zarr.codecs."):
	skip = True
	return skip

[normanrz]

I would also lean towards 1. Changing numcodecs would almost certainly also require some backwards-compat shim in zarr-python.

[jhamman]

Alright, let's do that. I'll close this and open a doc-only PR later today.

It's a shame because I quite liked how this shook out. But it is what it is is.