Mesh_3 - document example Weighted image + triple lines

[janetournois]

Summary of Changes

The example with 3D weighted images + detection of triple lines was not part of the documentation. This PR exposes it.

Release Management

• Affected package(s): Mesh_3 doc
• Related discussion : Techniques to smooth 3D mesh from labeled image #8285
• License and copyright ownership: unchanged

[sloriot]

Successfully tested in CGAL-6.0-Ic-300

[afabri]

/build:v0

[afabri]

I wanted to have a look at the documentation, but CGAL-6.0-Ic-300 is not listed on the testsuite page. So let's build it in the CI.

[github-actions]

The documentation is built. It will be available, after a few minutes, here: https://cgal.github.io/8373/v0/Manual/index.html

[afabri]

@janetournois don't we want a subsection in the User Manual for this example? Instead of blindly building the documentation I should have had a look at the "Files Changed" tab which shows that there is only an additional cpp.