docs: Pre-compute plugin_path before defining plugin

[MarcoGorelli]

the way the user guide currently shows, it re-computes plugin_path each time the plugin function is called (probably my fault from #14804)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79.86%. Comparing base (4dc90a9) to head (5f85c36).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #18503      +/-   ##
==========================================
- Coverage   79.87%   79.86%   -0.02%     
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  Files        1501     1501              
  Lines      202032   202032              
  Branches     2868     2868              
==========================================
- Hits       161370   161348      -22     
- Misses      40115    40137      +22     
  Partials      547      547              

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