Example `99-Open_an_existing` has a few problems

[mfisher87]

JupyterGIS version

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Description

1. This example indicates it's opening an existing project, but it actually creates a new project!
2. This example adds a GeoJSON layer to the project file that works but doesn't comply with the schema. See Adding a GeoJSON from Python API embeds it in the project file #1111 . This points at a larger problem of our architecture: A lot of behaviors are generated from the schema, but ultimately, we can take an escape hatch (see symbology) because the project file is not validated against the schema. This enables us to skip defining the schema accurately to what will actually show up in the project file.