Docs help! Add flowchart

[playaswd]

Can we add some flowcharts to illustrate the execution order of the pl functions or hooks for different best practices or application? Because i'm often confused about the fancy features.

1. Like manually opt for GAN, how to config lr schedule
2. log image every epoch should in train_step using is_last_batch but not in the train_epoch_end
   Thank you very much.