Improve testing

It's very hard to test and preview new changes with our new service: https://github.com/EclipseFdn/eclipsefdn-project-adopters/pull/31#issue-537071932

I think we can replace the Hugo site with a simple HTML page responsible for displaying new changes proposed in a PR.

We will need to fetch the content of adopters.json instead of making a request to our production API to include the new content in the preview.

I also think we need to add a test that validates the JSON file.

Please discuss!