Remove specs pages from docs

[dstansby]

These are just empty pages now, so I've replaced them with a link to the zarr specs on the home page. This cleans up the top level table of contents a little bit.

[d-v-b]

+1 for removing pages that are currently empty, but I think we will need a place in the docs to publish specs for things like consolidated metadata, dtypes, and codecs until the logjam over in zarr-specs clears up.

[dstansby]

we will need a place in the docs to publish specs

Do you think it's worth adding an empty page somewhere in this PR, or adding it later alongside the actual content?

[d-v-b]

lets wait for when we have actual content

[joshmoore]

I'm typically anti breaking any URLs. I'd suggest we redirect the spec/vX pages to the new homes and keep the top-level page also with the links to the new homes.