new meshes/solvers are stable

[ranocha]

I removed the experimental warnings and adapted the README.md accordingly. We should discuss whether the presentation of the main features in the README.md looks good to us in today's Trixi meeting.

Edit: The presentation of the main features is updated as discussed

Closes #542

[codecov]

Codecov Report

Merging #742 (7dc7709) into main (bd621f2) will not change coverage.
The diff coverage is n/a.

❗ Current head 7dc7709 differs from pull request most recent head c332c53. Consider uploading reports for the commit c332c53 to get more accurate results

@@           Coverage Diff           @@
##             main     #742   +/-   ##
=======================================
  Coverage   93.72%   93.72%           
=======================================
  Files         179      179           
  Lines       17082    17082           
=======================================
  Hits        16010    16010           
  Misses       1072     1072           

Flag	Coverage Δ
unittests	93.72% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
examples/dgmulti_2d/elixir_euler_weakform.jl	100.00% <ø> (ø)
examples/dgmulti_3d/elixir_euler_weakform.jl	100.00% <ø> (ø)
src/callbacks_step/analysis_dgmulti.jl	97.37% <ø> (ø)
src/meshes/dgmulti_meshes.jl	89.29% <ø> (ø)
src/meshes/p4est_mesh.jl	98.39% <ø> (ø)
src/meshes/structured_mesh.jl	90.53% <ø> (ø)
src/meshes/surface_interpolant.jl	100.00% <ø> (ø)
src/meshes/unstructured_mesh.jl	98.23% <ø> (ø)
src/solvers/dgmulti/dg.jl	96.27% <ø> (ø)
src/solvers/dgsem_p4est/containers.jl	92.57% <ø> (ø)
... and 2 more

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[jlchan]

LGTM now!

[efaulhaber]

LGTM!