Fixes #6425

#### What does this implement/fix? Explain your changes.
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This PR resolves the failure when trying to build docs
locally. As was shared on Discord
```
Notebook error:
JSONDecodeError in examples/00_sktime_intro.ipynb:
Expecting value: line 1 column 1 (char 0)
make: *** [html] Error 2
```

After deep inspection, the issue was the notebook that had extra
characters that weren't recognized by JSON decoder.
I added a fixed file and successfully tested the docs build locally from
my environment.