Improve documentation

[djdameln]

• Add documentation for dataset preparation/model preparation/model evaluation #258
• Add documentation for inference/export/optimization #259
• Add documentation for logging (tb/wandb)/HPO/benchmarking #260
• Add documentation to add a new model to the repo #304

[innat]

@samet-akcay In the discussion tab, the user can't create a new topic, the Net Discussion button is not showing. For example, a button like https://github.com/facebookresearch/detectron2/discussions

[samet-akcay]

@innat, looks like this is a setting in organizational level. I can't change it myself. Until we sort it out, you could create an issue and label as Discussion. I could then convert it to a discussion

[samet-akcay]

Addressed with #576.