Docs: Improve MudTreeView server data example by splitting off server data

[henon]

@dennisrahmen rightly noted that the treeview serverdata example is difficult to understand because we give the treeview a full tree in the example. Thus I split the initial top-level items off and made it so that the server load function searches from a separate tree hierarchy to demonstrate that the items are really loaded and not already there somehow.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.67%. Comparing base (28bc599) to head (8671a27).
Report is 280 commits behind head on dev.

Additional details and impacted files

@@            Coverage Diff             @@
##              dev    #9195      +/-   ##
==========================================
+ Coverage   89.82%   90.67%   +0.84%     
==========================================
  Files         412      400      -12     
  Lines       11878    12515     +637     
  Branches     2364     2436      +72     
==========================================
+ Hits        10670    11348     +678     
+ Misses        681      624      -57     
- Partials      527      543      +16     

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