Duplicate codec documentation

[dstansby]

Searching e.g., blosccodec in the v3 docs shows that codecs are be documented twice in the API docs.

I think the fix here is to clean up the codec namespacing, so all the public API is in zarr.codecs instead of the sub-modules. @normanrz do you think this is the right fix? I did try that in #2176, but I'm not sure why I closed it... I can have another go.

[normanrz]

Go for it!