Add a inv preview-site command

[fedepaol]

Is your feature request related to a problem?

When changing the docs, one has to download hugo (and make sure it's the right version!), move to the website folder and preview it.
It's not the end of the world but provides some friction to the contributor.

Describe the solution you'd like

A inv preview-site command to be added to the current set of tasks, that ensures hugo is present locally (or downloads it) and previews the website.

Additional context

No response

I've read and agree with the following

• I've checked all open and closed issues and my request is not there.
• I've checked all open and closed pull requests and my request is not there.