[ChrisW09]

Fixes #579

Problem

The observability tutorial placed the checkpoint of a plain fit() with default_root_dir. Without an ObservabilityConfig, DeepTab gives ModelCheckpoint an explicit dirpath under checkpoint_path (default model_checkpoints/, relative to the working directory), so default_root_dir has no effect. The tutorial wrote into the reader's working directory, the documented 01_no_observability/checkpoints/best_model.ckpt tree never appeared, and the notebook's own output said "(nothing was created here)".

Change

Docs only:

• docs/tutorials/observability.md: section 1 explains where the checkpoint goes and why default_root_dir doesn't move it, and calls train(checkpoint_path=str(WORKDIR / "01_no_observability" / "checkpoints")). The direct-logger fit in section 7b and the custom-sink fit in section 7c use checkpoint_path too; section 7b's text and tree now mention the checkpoint next to the logger's files. The trees in sections 1, 7b and 8 are now what the code produces.
• docs/tutorials/notebooks/observability.ipynb: the same changes in the matching markdown and code cells. The recorded outputs of the show_tree cells in sections 1 and 7b are updated to the trees the corrected calls print. I checked those outputs by running the two fits; I didn't re-execute the whole notebook.

Checks

• just test: 3236 passed, 46 skipped, 28 xfailed; coverage 91.29%
• just docs: build succeeded (warnings treated as errors)
• just check: all hooks passed (ruff, prettier, pyright)