Include /reference/ path on local builds

[LikeTheSalad]

I was checking the guide to contribute docs locally, and when I ran docs-builder serve I got no errors, but I couldn't see the docs by opening http://localhost:3000/ (I was getting 404 errors). Luckily after taking a look at some messages in some Slack groups I came across this other link: http://localhost:3000/reference/ which did the trick 👍 and I was thinking that we should update the guide to include the /reference/ path in this part to avoid confusions in the future.

An alternative solution mentioned here might be to show the full link in the console.

[reakaleek]

When #956 is released the CLI will redirect to the first page it finds if there is no root index.md file when running docs-builder serve.
In case of reference repos this means it will redirect to https://localhost:3000/reference