Broken netlify preview functionality

[andreslucena]

While checking out #129, I noticed that we have broken the netlify preview functionality settings in this repository.

Steps to reproduce:

1. Create a change in any file and commit it
2. Make a PR
3. Wait for the netlify deployment to finish
4. Click on "details" in the netlify preview check
5. See that you have the same as the production website (docs.decidim.org)

We need to also change the netlify preview functionality for actually checking the change against the branch that the person making the PR in the repository. As of now it doesn't work as it's making the preview deployment against develop (the branch configured in the playbook).

This is related to #131, so the fix would be similar:

The solution would involve having two playbook configurations, improving the documentation and explaining a bit how antora works, and having two commands: npm run build and npm run prod-build or something like that.

On this case it can be a bit trickier, as we'd need to check if the base branch is for the version branches (ie v0.27), but for now we can just fix develop as it's the 90% of the PRs, and we can tackle the version branches after that.

[andreslucena]

With the latest changes it's working. I just checked it out at #138.

Closing this issue.