Improve website structure to better present OpenRefine

[magdmartin]

This ticket follows this forum thread and focus on the first playbook named: About OpenRefine. The goal it to improve transparency on how the project is structured.

A first draft of the new website content and structure is available in this public Google Document (only section 1 About OpenRefine).

The proposed new layout is the following.

In the coming days, I will open a new branch and PR to add the new pages and change the structure. I will open separate tickets and PRs for the two other handbooks.

You can provide your feedback on this ticket, in the forum thread, or in the Google Doc as it suits you.