zarr-specs v3.1 follow-ons

[joshmoore]

These are issues I noticed while working on https://github.com/zarr-developers/zeps/pull/65 and related PRs, but didn't want

• [ ] remove references to now out of date issues (inline text if necessary) (prefer ZEP numbers?)
• [x] core > metadata > array: add subheaders "mandatory" and "optional"
• [x] c > m > a: additionally, add headers for "unknown" metadata and well as the example
• [ ] alphabetize glossary (concepts and terminologies)
• [ ] update gloassary figure to include extensions
• [ ] reduce duplication e.g. in chunk_grid and data_Type defining configuration
• [ ] unify "ASCII string" (Data types), "identifier", and "plain string"
• [ ] possibly extract chunk_key_encodings as a full extension point
  • https://github.com/zarr-developers/zarr-specs/pull/330#pullrequestreview-2626755764
• [ ] possibly extract node_type as a full extension point
• [ ] Add a section for chunk key encodings
• [ ] line 484 improve node_type definition https://github.com/zarr-developers/zarr-specs/pull/330#discussion_r1961710843
• [ ] lined 476 improve zarr_format definition https://github.com/zarr-developers/zarr-specs/pull/330#discussion_r1961709718