docs: fix errors and improve consistency across the documentation

[d-v-b]

I pointed claude fable at our docs and asked it to look for fixes

A full critical review of the documentation, with every finding verified by executing the runnable snippets against the current source (all tests/test_docs.py pass, strict mkdocs build is clean, and all relative links/anchors were mechanically checked).

Correctness fixes

performance.md had three claims contradicted by the page's own live-rendered output:

• The memory-order example claimed "Fortran order gives a better compression ratio" — in Zarr format 3, order affects in-memory layout only; stored bytes are identical (verified: both orders stored byte-identical data). The section is rewritten around what order actually does, with a note that serialized layout is governed by codecs.
• The write_empty_chunks conclusion was inverted: the emptiness check runs when the flag is False, so random-data writes are slower with False, not True (matches the measured timings the page renders).
• "Zarr's default concurrency of 64" → 10 (and the dependent arithmetic), and threading.max_workers "defaults to CPU count" → None; both now consistent with the correct statements earlier on the same page.
• The stale "MemoryStore cannot be pickled" claim is replaced with the real caveat (pickling copies the data).

Invisible code blocks: three exec="true" blocks without source="above" rendered as nothing in the built site (the rectilinear-chunks enable snippet in arrays.md, and the async.concurrency / threading.max_workers examples in performance.md). All are now visible.

Other errors:

• storage.md: the explicit-S3-filesystem example passed the bucket's virtual-hosted URL as endpoint_url (which would double the bucket name in request paths) and pointed FsspecStore at the filesystem root; now a correct copyable pattern.
• config.md: the "current default configuration" block showed order: 'F' because an earlier snippet in the same build session mutated the config; also codecs.bytes.name → codecs.bytes.
• arrays.md: "three configuration options" → all five ArrayConfig fields documented; memory:// URLs were attributed to MemoryStore (they resolve to an fsspec-backed store when fsspec is installed); wrong column indices in two comments; stray doctest-output line removed from data_types.md.
• v3_migration.md: Group.move and dimension_separator removed from the "not yet ported" lists (both are implemented); the truncated store-table row (a leftover from Improve store section in migration guide #3552) removed; the lapsed 2.x support window rewritten in past tense; the "🚧 Work in Progress 🚧" framing updated to 3.2.x reality.
• cli.md: the zarr[cli] extra is now documented (a plain install crashes with ModuleNotFoundError); the dry-run example output corrected to the real file:// form; the "equivalent functions" now name migrate_v2_to_v3/remove_metadata.
• installation.md: typing_extensions minimum corrected to 4.14; all five optional-dependency groups documented (the old text claimed universal_pathlib "must be installed separately" — it's the optional extra).
• gpu.md: zarr.Config.enable_gpu (nonexistent) → zarr.config.enable_gpu(); installation guidance added.
• contributing.md: docs build output is site/, not docs/_build/html; serve URL corrected; outdated doctest note removed.
• extending.md: custom data types are supported today (links to the dtype guide and worked example), not "in the future".
• quick-start.md: result="html" on a Python-repr block → result="code".

Usability improvements

• User Guide nav reordered into a beginner→advanced progression (data types moved next to the core concepts; the v3 migration guide moved out of the middle).
• quick-start.md: proper H1 and heading hierarchy, zarr[remote] note for the cloud example, and a "Next steps" section.
• user-guide/index.md now lists the CLI, experimental-features, and examples pages (previously undiscoverable from the guide index).
• groups.md: explains group/create_group/open_group, and documents member enumeration (keys, members, in) and deletion.
• attributes.md: documents update_attributes for bulk updates (each single assignment is a separate metadata write), deletion, and links consolidated metadata.
• consolidated_metadata.md: documents the public use_consolidated=True/False/None knob, including the escape hatch for the staleness caveat.
• glossary.md: adds Group, Attributes, and Consolidated Metadata entries; the Metadata entry now covers the v2 documents.
• Cross-links added between related pages throughout; the "Coming soon" stubs (rechunking, Blosc, custom stores) replaced with links to existing content.
• experimental.md: CacheStore sections properly nested; links the experimental API policy.
• release-notes.md: version headings and dates normalized.

Verification

• tests/test_docs.py: 57 passed, 2 skipped (GPU/cupy and a known skip).
• mkdocs build --strict: clean.
• All relative links and anchors mechanically verified.
• codespell: clean.

🤖 Generated with Claude Code

[d-v-b]

@zarr-developers/python-core-devs this is a big, flat, boring collection of AI-authored changes to our docs. If anyone wants to spot-check it, please have a look. Otherwise, I'm going to review everything by hand, and then merge shortly.