Link to specs for supported extensions

[d-v-b]

Should we add a new section to the docs that lists the zarr core features + extensions supported by zarr-python, and links to relevant specifications, when available? At the moment this would be:

• core dtypes (specified by the zarr spec)
• core codecs (specified by the zarr spec, or not, depending on how you read the text)
• forward-ported v2 dtypes (no specs written yet AFAIK)
• forward-ported v2 codecs (no specs written yet AFAIK)
• consolidated metadata (spec has been written here)

Ideally this content would be somewhere more centralized 🙃 , but for now the above proposal seems like a reasonable stop-gap measure.

[d-v-b]

for the v2 dtypes and codecs we could host the specs in this repo for the time being