Add geo-utils-cpp to Geospatial

[gistrec]

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

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

Inserted in Software → Geospatial, alphabetically between
Generic Mapping Tools (GMT) and geonotebook. Used the section's
existing entry format: en-dash separator, ![C++](media/icon/cplusplus.png)
language icon, short description ending in a period. No new section
needed; ToC unchanged. Searched for duplicates — none found.

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.

[JustinGOSSES]

Thanks for the contribution.