docs(datasets) Fix Flower Datasets tutorials naming and formatting

[adam-narozniak]

Issue

• the naming of tutorails is inconsistent
• the headers 1 in the tutorial give incorrect list in the index
• not all tutorials are listed in the tutorial list

Proposal

• rename the visualization how-to to tutorial (it's more a tutorial)
• change the headers 1 to 2 (and all the following inside one level down)
• update the links
• add missing tutorials to the index
• apply formatting