Build guide fixes
Build guide fixes

- Using nodejs-legacy to get rid of one build step
- Increased text size for "Step-by-step guide" header, as Tom@Huawei's feedback showed he got stuck when following simplified procedure
- Changed wording here and there to make things more understandable
loadimpact/k6,gbts/k6,StephenRadachy/k6,gbts/k6,loadimpact/k6,gbts/k6,StephenRadachy/k6,gbts/k6