Change documentation structure to have API and Developers docs at top level

[alexgarel]

Documentation is a bit messed up for API users because it's mixed with developer documentation.

While we want to follow the diataxis framework, we agreed we should split documentation, and flatten the structure.

• split in two main folders: API / Developer
• put all files flat under it but with fix prefix for files and titles based on the role of the documentation:
  • prefix in filename: how-to- / tutorial- / explanation- / reference-
  • prefix in titles: How to … / Tutorial: / Explanation: / Reference:
  • eventually an emoji in titles for each type of documentation (if it does not break the ordering)
• api.yml and all OpenAPI files deserve their own folder in API/reference