fix(docs): use label-based API version mapping in aztec-nr api page

[critesjosh]

Summary

Mirrors the fix from #22617 (against next) on the backport-to-v4-next-staging branch, but applied to the unversioned source since this branch does not yet have a v4.2.0 versioned snapshot.

Background

The Aztec.nr API reference page in v4.2.0 on next shipped with the pre-mainnet useApiVersion logic — links pointed to /aztec-nr-api/v4.2.0/... instead of /aztec-nr-api/mainnet/... and 404'd on the live site. The root cause on next was a stale versioned snapshot, already patched in #22617.

On this branch, docs-developers/docs/aztec-nr/api.mdx still has the same stale name-based useApiVersion:

const versionName = version?.name || "current";
if (versionName === "current") return "next";
if (versionName.includes("rc") || versionName.includes("testnet")) return "testnet";
return versionName;  // ← v4.2.0 would fall through here

If a mainnet version gets cut from this branch, the new snapshot would inherit the same bug. Pre-empting that here.

Fix

Replace useApiVersion with the label-based mapping that already lives on next:

if (!version || version.name === "current") return "next";
const label = version.label || "";
if (label.startsWith("Alpha")) return "mainnet";
if (label.startsWith("Testnet")) return "testnet";
return version.name;

The Alpha (...) label comes from docusaurus.config.js for mainnet-mapped versions, so useApiVersion() resolves to "mainnet" for any future v4.2.0+ snapshot cut from this branch.

Test plan

• Run yarn start locally on this branch and verify the Aztec.nr API Reference page renders without broken links on the current (dev) version.
• When the next mainnet cut lands, confirm the resulting snapshot inherits the label-based logic.

[critesjosh]

handled by #22622