## 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:
- [x] Update usages of now deprecated PMP border.h (moved to BGL)
- [x] Test deprecated border.h
- [x] Fix "Topics" order
- [x] Dispatch functions and doc coming from:
    - [x] #9131
      - [ ] use the new make_hex in add_bbox
- [x] Move .h but keep include/CGAL/PMP

Tentative TODO:
- [ ] Update some package icons?

## Release Management

Integrate after :
- #8935
- #9131

* Affected package(s): `PMP`
* Issue(s) solved (if any): 
* Feature/Small Feature (if any):
[PMP_doc_breakup](https://cgalwiki.geometryfactory.com/CGAL/Members/wiki/Features/Small_Features/PMP_doc_breakup)
* Link to compiled documentation (obligatory for small feature) [*wrong
link name to be changed*](httpssss://wrong_URL_to_be_changed/Manual/Pkg)
* License and copyright ownership: no change