Doc refactor (#311)

docs/dataset_prepare.md

docs/get_started.md

docs/index.rst

docs/inference.md

docs/train.md

docs/tutorials/config.md

docs/tutorials/customize_datasets.md

docs/tutorials/customize_models.md

docs/tutorials/customize_runtime.md

docs/tutorials/data_pipeline.md

docs/tutorials/index.rst

docs/tutorials/training_tricks.md

docs/useful_tools.md

tools/convert_datasets/pascal_context.py