DOC: Rewrite 'remove-empty-intervals' tutorial to be an example of an end-to-end analysis with crowsetta

[NickleDave]

• Rewrite to just be an example of an end-to-end analysis
• Add the TextGrid files from the dataset that parse without error to repo, instead of downloading of OSF, to avoid download error and make tutorial shorter
• Rewrite empty intervals language to explain it's done by default