Update local development pre-requisites

[immavalls]

With the recent changes, we can now run the docs locally if npm has already been installed.

We need to update the docs to mention that this is optional. If npm is installed run npm start,. If not, and we only have docker/podman, continue running cd docs && make docs.

Worth mentioning that with legacy docs we can still run them locally as in https://github.com/grafana/k6-docs/tree/addExperimentalTimers?tab=readme-ov-file#install-and-run-in-docker. Might be worth keeping a separate page about how to test the legacy docs linked from https://github.com/grafana/k6-docs/blob/main/CONTRIBUTING/README.md#build-locally?