DOCS: Delete v4 api page

This page was created during the initial v4 hackathon and hasn't been updated much since and is no longer accurate. Let's remove as it has served its purpose in planning.

Author: VeckoTheGecko

docs/v4/api.md

@@ -18,7 +18,6 @@ The pages below provide further background on the development of Parcels v4. You
 
 ```{toctree}
 installation
-api
 nojit
 TODO
 Parcels v4 Project Board <https://github.com/orgs/Parcels-code/projects/5>