fix(docs): backport #22617 — label-based API version mapping (v4-next) (#22622)

Backport of #22617 — *fix(docs): restore Aztec.nr API reference links on
v4.2.0* — to `v4-next` via `backport-to-v4-next-staging`.

## Why the automatic cherry-pick failed

The upstream fix modifies
`docs/developer_versioned_docs/version-v4.2.0/docs/aztec-nr/api.mdx`,
which does **not exist on v4-next**. Git's rename detection landed the
diff on `version-v4.1.0-rc.2/docs/aztec-nr/api.mdx` with conflict
markers — the wrong target.

On v4-next, the equivalent stale `useApiVersion` logic lives in
`docs/docs-developers/docs/aztec-nr/api.mdx`. That file is the source
from which future versioned snapshots are cut, so without this fix the
same regression would recur the moment v4.2.0 developer docs are cut
from v4-next.

## Commit structure

Per `CLAUDE.md` backport guidance:

1. `cherry-pick: ... (with conflicts)` — mechanical cherry-pick
committed AS-IS for history.
2. `fix: resolve cherry-pick conflicts by reverting wrong target` —
restores `version-v4.1.0-rc.2/...` (whose existing name-based logic
already resolves correctly for its `Testnet (v4.1.0-rc.2)` label, since
the name matches `includes("rc")`).
3. `fix(docs): use label-based API version mapping in docs-developers
aztec-nr api page` — applies the same 7/-5 change from #22617 to the
real v4-next target.

## Net diff

```
docs/docs-developers/docs/aztec-nr/api.mdx | 12 +++++++-----
1 file changed, 7 insertions(+), 5 deletions(-)
```

Full analysis:
https://gist.github.com/AztecBot/4994eafc80a7cc7081ecf215fc4a9aec

## Test plan

- [ ] `yarn start` locally on v4-next, open the Aztec.nr API Reference
under the Alpha version, confirm links resolve.
- [ ] Verify the next v4.2.0 versioned cut from v4-next inherits the
fixed logic.

ClaudeBox log: https://claudebox.work/s/056d3a595d4c0d37?run=1

Author: critesjosh

docs/docs-developers/docs/aztec-nr/api.mdx

@@ -10,11 +10,13 @@ import { useActiveVersion } from "@docusaurus/plugin-content-docs/client";
 
 export const useApiVersion = () => {
   const version = useActiveVersion("developer");
-  const versionName = version?.name || "current";
-  // Map Docusaurus version to API docs folder
-  if (versionName === "current") return "next";
-  if (versionName.includes("rc") || versionName.includes("testnet")) return "testnet";
-  return versionName;
+  if (!version || version.name === "current") return "next";
+  // Map Docusaurus version to API docs folder using version label
+  // Labels are set explicitly in docusaurus.config.js (e.g., "Mainnet (...)")
+  const label = version.label || "";
+  if (label.startsWith("Alpha")) return "mainnet";
+  if (label.startsWith("Testnet")) return "testnet";
+  return version.name;
 };
 
 export const ModuleLink = ({ path, children }) => {