update examples and docs

[wzever]

1. update the way of setting backend using pygm.set_backend()
2. update the way of fetching example data using pygm.benchmark.Benchmark
3. resolve problems and package versions for docs generation.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (dbfdad9) 92.84% compared to head (a13b756) 92.84%.
Report is 4 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #88   +/-   ##
=======================================
  Coverage   92.84%   92.84%           
=======================================
  Files          20       20           
  Lines        7600     7617   +17     
=======================================
+ Hits         7056     7072   +16     
- Misses        544      545    +1     

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[rogerwwww]

Nice work! merged