doc: add design decisions document

doc/design-decisions.md

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+# Design Decision
+## Communication between Browser and Server via REST-like Endpoints and HTMX
+- https://htmx.org/essays/10-tips-for-ssr-hda-apps/
+- Name of endpoints is plural
+    - "/locations" GET -> read all
+    - "/locations/new" GET / POST -> create
+    - "/locations/1" GET -> edit, POST -> update, DELETE -> delete CRUD
+    - "/locations/assign" GET / POST for custom action
+- Modelling of models in code
+    - see vehicle.rs
+    - use of applicationError
+    - return Options, where queries are bound to id or search input