Add performance page

[ianmackenzie]

• Discuss how future versions of elm-geometry will improve performance by manually inlining code/avoiding allocations
• Describe best practices
  • Using relativeTo/placeIn for composite transformations (or possibly upcoming transformation support)
  • Extracting coordinates/components without allocations/function calls, by destructuring
  • Using fromCoordinates instead of fromTuple/fromRecord, and fromCoordinatesIn instead of fromCoordinates/placeIn, to avoid unnecessary allocations