Add open boundaries

[LasNikas]

based on #437 and #440

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 65.97353% with 180 lines in your changes missing coverage. Please review.

Project coverage is 68.68%. Comparing base (39bc798) to head (b0d6563).

Files	Patch %	Lines
src/schemes/boundary/open_boundary/system.jl	59.43%	86 Missing :warning:
src/visualization/write2vtk.jl	0.00%	24 Missing :warning:
...st/schemes/boundary/open_boundary/boundary_zone.jl	86.00%	14 Missing :warning:
src/callbacks/update.jl	0.00%	9 Missing :warning:
src/general/semidiscretization.jl	50.00%	7 Missing :warning:
src/general/system.jl	40.00%	6 Missing :warning:
...c/schemes/boundary/open_boundary/boundary_zones.jl	91.66%	6 Missing :warning:
...rc/schemes/fluid/entropically_damped_sph/system.jl	44.44%	5 Missing :warning:
src/general/buffer.jl	90.47%	4 Missing :warning:
src/general/density_calculators.jl	0.00%	4 Missing :warning:
... and 8 more

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #442      +/-   ##
==========================================
+ Coverage   68.56%   68.68%   +0.12%     
==========================================
  Files          69       76       +7     
  Lines        4040     4548     +508     
==========================================
+ Hits         2770     3124     +354     
- Misses       1270     1424     +154     

Flag	Coverage Δ
unit	68.68% <65.97%> (+0.12%)	:arrow_up:

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[svchb]

Add to news.md and update examples doc page.

[svchb]

Update NEWS.md and README.md. Also add pictures to the docs for the examples or add it to the list here #495.