refactor(registry): add mf6 prefix to mf6 model names

Author: wpbonelli

docs/md/models.md

@@ -59,8 +59,10 @@ The script can be executed with `python -m modflow_devtools.make_registry`. It a
 For example, to create a registry of models in the MF6 examples and test models repositories, assuming each is checked out next to this project:
 
 ```shell
-python -m modflow_devtools.make_registry ../modflow6-examples/examples --url https://github.com/MODFLOW-ORG/modflow6-examples/releases/download/current/mf6examples.zip --prefix example
-python -m modflow_devtools.make_registry ../modflow6-testmodels/mf6 --append --url https://github.com/MODFLOW-ORG/modflow6-testmodels/raw/master/mf6 --prefix test
-python -m modflow_devtools.make_registry ../modflow6-largetestmodels --append --url https://github.com/MODFLOW-ORG/modflow6-largetestmodels/raw/master --prefix large
+python -m modflow_devtools.make_registry ../modflow6-examples/examples --url https://github.com/MODFLOW-ORG/modflow6-examples/releases/download/current/mf6examples.zip --prefix mf6/example
+python -m modflow_devtools.make_registry ../modflow6-testmodels/mf6 --append --url https://github.com/MODFLOW-ORG/modflow6-testmodels/raw/master/mf6 --prefix mf6/test
+python -m modflow_devtools.make_registry ../modflow6-largetestmodels --append --url https://github.com/MODFLOW-ORG/modflow6-largetestmodels/raw/master --prefix mf6/large
 python -m modflow_devtools.make_registry ../modflow6-testmodels/mf5to6 --append --url https://github.com/MODFLOW-ORG/modflow6-testmodels/raw/master/mf5to6 --prefix mf2005 --namefile "*.nam"
-```
+```
+
+Above we adopt a convention of prefixing model names with the model type (i.e. the program used to run it), e.g. "mf6/" or "mf2005/". Relative path parts below the initial prefix reflect the model's relative path within its repository.