Add sorting, selection, lazy loading and styling examples for TreeGrid

[juuso-vaadin]

All the current docs examples have two levels of hierarchy in the data. Having an enhanced example with 3 or 4 levels could clarify these cases:

• How sorting works on hierarchical dataset?
• Is multi column sorting still available?
• How multi-select mode works?
• Lazy-loading functionality and configuration options
• Can the rows be styled based on level?

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