DOC: Add a link to Mayavi example for mesh decimation

[clementkng]

Description

This commit adds a link to Mayavi example for mesh decimation in the marching_cubes docstring.

Temporarily addresses #4537.

Checklist

• Docstrings for all functions
• Gallery example in ./doc/examples (new features only)
• Benchmark in ./benchmarks, if your changes aren't covered by an
  existing benchmark
• Unit tests
• Clean style in the spirit of PEP8

For reviewers

• Check that the PR title is short, concise, and will make sense 1 year
  later.
• Check that new functions are imported in corresponding __init__.py.
• Check that new features, API changes, and deprecations are mentioned in
  doc/release/release_dev.rst.

[emmanuelle]

Thanks @clementkng this is helpful and looks good to me! Good luck with the decimation algorithm now :-).

[soupault]

Thank you @clementkng !