Add docs page for switching from other geometry libraries

[DmitryLambin]

Summary

Adds a top-level documentation page Switching from Other Libraries to MeshLib (meshlib.io/documentation/SwitchingFromOtherLibraries.html).

Developers evaluating MeshLib usually arrive with an existing CGAL / libigl / Open3D / PCL / VTK pipeline, and their first question is how their current calls map onto ours. The docs had no page answering that. This one pairs the most frequently ported operations with the equivalent MeshLib code:

Library	Operations covered
CGAL	boolean union, connected components
libigl	decimation, signed distance
Open3D	point-to-point ICP, point cloud → mesh
PCL	voxel-grid downsampling, normal estimation
VTK	decimation, boolean union, mesh IO, in-memory numpy bridge (Python)

Two files:

• doxygen/general_pages/SwitchingFromOtherLibraries.dox — the page (23 code blocks, no images).
• doxygen/layout_templates/base_struct.xml — one nav entry, top-level, between Examples and Package Overview.

Notes for review:

• Every MeshLib snippet was checked against the current headers in source/ (includes, signatures, Expected/optional handling). Competitor snippets are illustrative and are not built by CI — that is the same trade-off the page's copy makes explicit ("copy, adapt, compile").
• The page does not link to any competitor site and carries no benchmark numbers; the only outbound links are our own blog article and this repo's issues/discussions.
• Content approved by the marketing owner (task GEN-3715); this PR is the publication step.
• To appear in Google, SwitchingFromOtherLibraries.html also needs a line in whitelist.txt of MeshInspector/MeshInspector.github.io (the rest of the docs is googlebot: noindex). Not part of this repo — flagged separately.

Test plan

Built the Main docs module locally with doxygen against this branch and against origin/master, using the real DoxyfileMain from MeshInspector.github.io:

• SwitchingFromOtherLibraries.html is generated; title, all 6 library sections and 13 sub-sections appear in the page TOC; 23 code blocks render.
• The nav entry appears in both menudata.js (top menu) and navtreedata.js (sidebar) pointing at SwitchingFromOtherLibraries.html.
• Warning diff branch vs master: 138 vs 138, identical set — no new doxygen warnings.
• All three outbound links return HTTP 200.