Documentation: Add comparison to A5

[garikAsplund]

A5, an open source, global geospatial index that uses irregular equilateral pentagons for higher accuracy and lower distortion.

[DoiMayank]

Hi @garikAsplund , is one liner enough ?

H3 is a hierarchical hexagonal spatial index optimized for performance and scalability, while A5 is a pentagon based global index designed to reduce distortion with a focus on geometric accuracy.