Proposal: split documentation into multiple pages following Diátaxis #122

Hello !

I wish to contribute to the OCHA DAP project, because I want to support this kind of initiative. I noticed some recent activity in this repo and figured I could update the documentation as a start. I am happy to contribute elsewhere if more relevant instead.

All the user documentation is currently stored in documentation/index.md which makes it difficult to find specific information and the tutorial, reference and how-to guide are mixed together.

I suggest to split the documentation/index.md file into different files following the Diátaxis framework. If you are following other documentation styles for the HDX libraries, I could follow it as well but could not find a specific guideline.

I would split documentation/index.md into 4 different categories without editing the content (Tutorials, How-To Guides, Reference, Explanation). This might break links used elsewhere; is it ok if existing URLs/anchors change, or should I add redirects (e.g. a redirects plugin)? I am happy to take on other issues afterwards.

Best,
Antonin