Improve Documentation website

[swissiety]

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

[github-actions[bot]]

Documentation Preview.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

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

:exclamation: 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

```diff @@ 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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