Improve Documentation website

[swissiety]

• improved design (hopefully)
• restructured Jimple
• elaborated a lot of sections
• add examples for other Visitors than the StmtVisitor, too

[github-actions]

Documentation Preview.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 70.17%. Comparing base (f6d87d6) to head (610c531).
Report is 19 commits behind head on develop.

❗ Current head 610c531 differs from pull request most recent head fbe539b

Please upload reports for the commit fbe539b to get more accurate results.

Additional details and impacted files

@@              Coverage Diff              @@
##             develop     #955      +/-   ##
=============================================
- Coverage      70.72%   70.17%   -0.56%     
+ Complexity      4141     4130      -11     
=============================================
  Files            314      315       +1     
  Lines          15521    15588      +67     
  Branches        2663     2672       +9     
=============================================
- Hits           10978    10939      -39     
- Misses          3658     3762     +104     
- Partials         885      887       +2     

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