Add a preview to the site on CI with a clickable link to visually inspect site updates

**Is your feature request related to a problem? Please describe.**
When changes occur on the site, there is currently no option to visually inspect directly on a browser.

It'd be good to have a preview option in the CI to avoid checkout the branch and serve the site locally to visually check website updates.

Currently, in order to run the site locally we need the following:

```sh
# from retina project root
make docs-prod
cd site
npm run serve
```

**Describe the solution you'd like**
An automated step included in the CI that provides a clickable link with the site preview based on the most recent PR commit

**Describe alternatives you've considered**
An alternative is to add clear instructions under https://retina.sh/docs/Contributing/developing with how to serve the site locally.

**Additional context**
Some OpenSource projects use netlify, see https://github.com/istio/istio.io

FYI @nddq @ibezrukavyi