Restructure docs, clarify extension points and configs, use short ids

[jstriebel]

This PR

• restructures the headers to integrate other zarr pages,
• adds links to v2 and v1 specs,
• clarifies extension points, fixes Why do storage transformers need "type" separate from "configuration" #191:
  • future features will be added as new top-level keys
  • extension points have coherent configs
  • updated the stability policy accordingly
• changes all identifiers to be simple short forms, fixes v3 core spec: purl urls as identifiers #176,
• adds minor cleanups, e.g. removing resolved questions.

Preview of the built docs:
https://zarr-specs--204.org.readthedocs.build/en/204/v3/core/v3.0.html

[jbms]

Note: complex numbers are now part of the main spec so we can remove that extension.

[jstriebel]

@jbms Thanks for the review! I applied your feedback, and also updated the sharding and codecs spec to the newest format. This is also necessary for the data-types, but I think those need to be partially re-written anyways, not only with the new configuration syntax. It would be great to have somebody authoring a ZEP for those. (cc @MSanKeys963, might be something to ask around for in the next community meeting?)

Also cc @joshmoore and @MSanKeys963, what do you think of the updated menu header? I could imagine to use the same headers with external links on multiple pages, to get the impression of a single coherent page. This doesn't need to be tackled in the specs first, but I thought this could be a good opportunity to play around with this idea, that's why I added it here.

[jbms]

Aside from the typos, looks good to me. We may want to iterate more, but easier if this is submitted first, I think.

[rabernat]

This looks good to me overall. Thanks for doing this! 🙌