docs: link the zarr-http-server docs site, and fix dead links across subpackage READMEs

[d-v-b]

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Adds the Read the Docs link to the zarr-http-server README, rebalances README vs docs site to match the sibling packages, and fixes two dead links found by checking every URL across all three subpackages.

Why the rebalance

The README was 405 lines while the docs site was a 58-line scaffold — inverted relative to its siblings, and with no link to the site at all:

	README	docs/index.md
zarr-metadata	134	98
zarr-indexing	52	53
zarr-http-server (before)	405	58
zarr-http-server (after)	71	71 + 393 (guide)

Documentation: <https://zarr-http-server.readthedocs.io/> now sits at the top, where both siblings put theirs.

The detailed sections became docs/guide.md: building apps, running them, serving several nodes, notebooks, Uvicorn configuration, CORS, byte ranges, read-only serving, writes, shutdown. Nothing was dropped — the README keeps what someone skimming PyPI needs: what it is, install, a quick start, the build/run split, and the warning that store_app filters nothing.

Dead links fixed

Found by checking every URL in all three packages' READMEs and docs, not just the file being edited:

• zarr-metadata's Documentation: link 404s. It points at readthedocs.io/en/stable/, and neither zarr-metadata nor zarr-indexing has a stable version. Dropped the version segment so it matches the other two and lets Read the Docs redirect to the default.
• www.uvicorn.org no longer resolves at all — the domain is gone, so it fails DNS rather than returning a 404, which is why it survives most link checkers. Uvicorn's own PyPI metadata now gives https://uvicorn.dev/. Fixed in the two places it appeared.

Accuracy fixes, not moved verbatim

Reviewing before moving turned up several things that were wrong:

• docs/index.md paired store_app with serve and node_app with serve_background as though they were coupled. They are independent — either app runs under either runner. Collateral from the serve_store/serve_node → serve/serve_background rename.
• docs/index.md sent readers to the README for worked examples. Backwards now, and would have been a loop.
• The range section cited RFC 7233, obsoleted by RFC 9110, while the rest of the same section cited 9110.
• Write examples used a bare methods={"GET", "PUT"} although the read-only section a few paragraphs earlier introduced READ_WRITE_HTTP_METHODS for exactly that.
• "Read-only serving" and "Write support" restated each other; merged.
• The API reference documented 8 of 13 public names. Added ReadOnlyHTTPMethod, READ_ONLY_HTTP_METHODS, READ_WRITE_HTTP_METHODS, AUTO_PORT, DEFAULT_PORT. The missing ReadOnlyHTTPMethod was also breaking a cross-reference from the new guide, which --strict caught.

Claims that were kept got re-verified against the code — DEFAULT_MAX_BODY_SIZE is 256 MiB, PUT within the cap returns 204 and over it 413, an unsupported method raises ValueError at construction.

Link form

Deep links use /en/latest/, verified to resolve against an existing page. /en/stable/ was avoided precisely because it is what broke zarr-metadata.

Verification

just check passes: ruff, mypy, 212 tests, and a --strict docs build with the new page in the nav. Every link in all three READMEs and docs trees resolves, except this PR's own links to the not-yet-published guide page, which resolve once the site rebuilds.