Minor improvements when preparing WAVES talk

[maltezfaria]

• Make the returned type of assemble_single_double_layer more uniform by returning a linear map even if it is possible to return an HMatrix
• Some helper functions for meshes
• Add logo to docs
• Simple and still incomplete example of how to create a graded mesh for a scattering problem with corners

[codecov]

Codecov Report

Attention: Patch coverage is 8.33333% with 11 lines in your changes missing coverage. Please review.

Project coverage is 65.90%. Comparing base (399d4ab) to head (f46889d).

Files	Patch %	Lines
src/utils.jl	0.00%	6 Missing ⚠️
src/api.jl	0.00%	2 Missing ⚠️
src/vdim.jl	0.00%	2 Missing ⚠️
src/mesh.jl	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #76      +/-   ##
==========================================
- Coverage   65.99%   65.90%   -0.09%     
==========================================
  Files          25       25              
  Lines        2938     2942       +4     
==========================================
  Hits         1939     1939              
- Misses        999     1003       +4     

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