Next.js example implement preview mode

[fmontes]

Is your feature request related to a problem? Please describe. Tipically users that create content want to see how that content looks like in their site before pressing "publish" button.

Out of the box Next.js will always fetch the dotCMS live data to build the pages but some cases (preview) we need to fetch "working data"

Describe the solution you'd like Next.js have a solution for this and is "preview mode", all the information here

In short, when preview mode is true we need to build the page using the working version of the contentlet in dotCMS.

Additional context We need to find a way to provide the best content editor experience as well as the best developer experience in this approach.