Add geo-utils-cpp to Geometry section

[gistrec]

Adds geo-utils-cpp — a C++17 library for spherical (lat/lng) geometry on Earth coordinates: distance, bearing, area, point-in-polygon, path proximity.

• License: Apache-2.0
• Header-only, no runtime dependencies
• C++17, CMake 3.14+
• CI on Linux (gcc/clang), macOS, Windows; downstream find_package smoke-tested on all three

Inserted alphabetically in the Geometry, Graphics Processing, and Game Development section between entt and glm. Adjacent to earcut.hpp (Mapbox's polygon triangulation) which serves a similar geometry niche.

Followed CONTRIBUTING.md:

• Searched for duplicates — no existing entry
• Single suggestion per PR
• Description is one rendered line, ends with period, no "header-only" phrasing
• Inserted in alphabetical order within the category

Note: this PR was originally opened as geo-utils; the project was
renamed to geo-utils-cpp to avoid a Repology naming conflict
(see gistrec/geo-utils-cpp#11). The branch has been force-pushed
to use the new name.