Make help dialog content paths compatible with various OSes

[Sleet01]

Compiles various help file paths into MMConstants and wraps them in calls to Paths.get().toString() for wider compatility.

Testing:

• Opened the help dialogs in MegaMek
• Ran all 3 projects' unit tests

Close #4301

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 29.07%. Comparing base (61fa436) to head (3c591be).
Report is 1 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #5767      +/-   ##
============================================
- Coverage     29.07%   29.07%   -0.01%     
+ Complexity    13908    13907       -1     
============================================
  Files          2507     2507              
  Lines        266290   266309      +19     
  Branches      47598    47598              
============================================
- Hits          77420    77418       -2     
- Misses       184951   184973      +22     
+ Partials       3919     3918       -1     

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