Add docsite preview to CI

[martriay]

The docs-utils project suggests a javascript-based solution to implement docsite previews on PRs. This issue aims to adapt it to this repository for the relevant PRs.

[frangio]

I think we can still use docs-utils just using npx so we don't have to add a package.json.

[frangio]

Expanding on the above: as long as Node.js is installed, npx is available and can be used to run docs-utils without installing it. It would look something like npx @openzeppelin/docs-utils -- -c docs (-- may or may not be necessary).

On GitHub Actions you need to add use: actions/setup-node.

[martriay]

fixed by #459