Docs: Add another level of depth to the MudTreeView ServerData example

[henon]

The tree in the server loading example is quite shallow. Added a third level to it for demonstration purposes. Also removed some unnecessary clutter that adds nothing of value to the example. The didactic value should be much greater now.

@dennisrahmen FYI

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.69%. Comparing base (28bc599) to head (9e1eca9).
Report is 277 commits behind head on dev.

Additional details and impacted files

@@            Coverage Diff             @@
##              dev    #9192      +/-   ##
==========================================
+ Coverage   89.82%   90.69%   +0.86%     
==========================================
  Files         412      400      -12     
  Lines       11878    12514     +636     
  Branches     2364     2436      +72     
==========================================
+ Hits        10670    11349     +679     
+ Misses        681      623      -58     
- Partials      527      542      +15     

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