Add a markdown landing page in codespaces

[scottyhq]

Launching into codespaces currently brings up a fairly blank vscode canvas:

If you're familiar with the interface it works well (e.g. ASP commands just work in the terminal)! But I think it would be good to add a markdown 'index' file that suggests trying some commands to get started or that links to the different notebooks. See how this is done in the Xarray tutorial here https://github.com/xarray-contrib/xarray-tutorial/blob/8bcdd9f23757446a3acb6048a5cd00e1db6852df/.devcontainer/devcontainer.json#L9-L11

[dshean]

Seems like a better option.

Or maybe it would be possible to open and display the example aster notebook by default, where the instructions are in the first few cells?

Maybe some re-org would be useful, moving the geojson and asp_binder_utils.py to a different subdirectory, like assets?