[Documentation] COG resolution / Overview -> min/max zoom level

[vincentsarago]

ref:
#290
#287

Let's write a documentation that explains how rio-tiler/titiler define Min/Max zoom for particular COG. Also Recently we had question about Mosaic performance and comparison with VRT. The performance of Mosaic or VRT will all be linked to the zoom level (the lower zoom level you request a tile the more file you'll need to fill the tile)