[docs-new-contributor] New contributor docs review — Go setup version is outdated

## Summary
The primary contributor onboarding path currently tells new contributors to install Go `1.22.10`, but this repository now requires Go `1.24.13`. This mismatch is in the first setup steps and can block or mislead first-time contributors.

## Findings

### 1. Go version in setup docs does not match repository requirement

**Where:** `docs/extend/index.md` → "Setting Up Your Dev Environment" (line 27), and linked from `CONTRIBUTING.md` (lines 14-16)

**Problem:** The doc states: "install the 1.22.10 version of Go". However, `go.mod` declares `go 1.24.13` (line 3).

**Impact:** New contributors following the documented prerequisite can start with the wrong Go version and hit avoidable setup/build friction before making any code changes.

**Suggested fix:** Update `docs/extend/index.md` to use the current required Go version (or reference `.go-version` / `go.mod` to avoid future drift), and ensure `CONTRIBUTING.md` still points to the updated onboarding step.

## Suggested Actions

- [ ] Update the Go version prerequisite in `docs/extend/index.md` to match `go.mod`.
- [ ] Replace hardcoded Go version text with a single source of truth reference to prevent future mismatch.

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