Breakup and improve PMP's documentation

[MaelRL]

Summary of Changes

With the years and the additions to the package "Polygon Mesh Processing" (PMP), the user and reference manuals have grown overly large, making it difficult to find information.

This PR splits the documentation of PMP into four (three new) packages:

• PMP (core): essential functions, ranging from simple geometric functions to distance functions, feature detection etc.
• Boolean Operations on Meshes: Corefinement, autorefinement, clipping, splitting, slicing, [upcoming kernel ?]
• Meshing and Remeshing of Polygon Meshes: combinatorial and geometric, local and global meshing and remeshing.
• Polygon Mesh Repair: detection and treatment of defects in polygon meshes

TODO:

• Update usages of now deprecated PMP border.h (moved to BGL)

Release Management

Integrate after :

• [PMP] Approximate convex decomposition #8935
• Add experimental functions to compute the kernel of a triangle mesh #9131

• Affected package(s): PMP
• Issue(s) solved (if any):
• Feature/Small Feature (if any):
• Link to compiled documentation (obligatory for small feature) wrong link name to be changed
• License and copyright ownership: no change

[MaelRL]

/force-build:v0

[github-actions]

There was an error while building the doc:

<unknown>:12: warning: Detected potential recursive class relation between class CGAL::Polygon_mesh_processing::Corefinement::Non_manifold_output_visitor and base class CGAL::Polygon_mesh_processing::Corefinement::Default_visitor!
<unknown>:12: warning: Detected potential recursive class relation between class CGAL::Polygon_mesh_processing::Corefinement::Non_manifold_output_visitor and base class CGAL::Polygon_mesh_processing::Corefinement::Default_visitor!
<unknown>:12: warning: Detected potential recursive class relation between class CGAL::Polygon_mesh_processing::Corefinement::Non_manifold_output_visitor and base class CGAL::Polygon_mesh_processing::Corefinement::Default_visitor!
/home/runner/work/cgal/cgal/Polygon_mesh_processing/include/CGAL/Polygon_mesh_processing/triangulate_faces.h:47: warning: Detected potential recursive class relation between class CGAL::Polygon_mesh_processing::Triangulate_faces::Default_visitor and base class CGAL::Polygon_mesh_processing::Hole_filling::Default_visitor!
/home/runner/work/cgal/cgal/Polygon_mesh_processing/include/CGAL/Polygon_mesh_processing/triangulate_faces.h:465: warning: Detected potential recursive class relation between class CGAL::Polygon_mesh_processing::Triangulate_polygons::Default_visitor and base class CGAL::Polygon_mesh_processing::Hole_filling::Default_visitor!

https://github.com/CGAL/cgal/actions/runs/21102033181