Add day3 main material

day3/ProgrammingInCXX_d3.pdf

day3/examples/gcd_w_concepts.cc

day3/examples/generic_func1.cc

day3/examples/generic_func2.cc

day3/examples/generic_func3.cc

day3/examples/generic_func4.cc

day3/examples/geometry/CMakeLists.txt

day3/examples/geometry/Circle.cc

day3/examples/geometry/Circle.hh

day3/examples/geometry/Point.cc

day3/examples/geometry/Point.hh

day3/examples/geometry/Polygon.hh

day3/examples/geometry/Shape.hh

day3/examples/geometry/Triangle.cc

day3/examples/geometry/Triangle.hh

day3/examples/geometry/main.cc

day3/examples/ipsum.txt

day3/examples/lambda_captures.cc

day3/examples/mutable_lambdas.cc

day3/examples/overload_w_concepts.cc