doc: drop dev-notes & make update-dbix more discoverable

`dev-notes` are severely outdated. I dropped everything except one note
that I moved to hacking.md. The parts about creating users are also
covered elsewhere.

The `update-dbix` part got a just command to make it discoverable again.

src/sql/hydra.sql

@@ -6,7 +6,7 @@
 --    add a map of the lowercase name of your table to the CamelCase
 --    version of your table.
 --
--- 3. Run `make -C src/sql update-dbix` in the root
+-- 3. Run `just update-dbix` in the root
 --    of the project directory, and git add / git commit the changed,
 --    generated files.
 --