zarr-specs v3.1 follow-ons

[joshmoore]

These are issues I noticed while working on zarr-developers/zeps#65 and related PRs, but didn't want

• remove references to now out of date issues (inline text if necessary) (prefer ZEP numbers?)
• core > metadata > array: add subheaders "mandatory" and "optional"
• 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
  • ZEP9 (phase 1): add clarifications for extension naming #330 (review)
• possibly extract node_type as a full extension point
• Add a section for chunk key encodings
• line 484 improve node_type definition ZEP9 (phase 1): add clarifications for extension naming #330 (comment)
• lined 476 improve zarr_format definition ZEP9 (phase 1): add clarifications for extension naming #330 (comment)