Add markdown lint in pre-commit hook (#4255)

.github/CONTRIBUTING.md

.github/ISSUE_TEMPLATE/error-report.md

.github/ISSUE_TEMPLATE/reimplementation_questions.md

.pre-commit-config.yaml

README.md

configs/atss/README.md

configs/cascade_rcnn/README.md

configs/centripetalnet/README.md

configs/cityscapes/README.md

configs/cornernet/README.md

configs/dcn/README.md

configs/deepfashion/README.md

configs/double_heads/README.md

configs/empirical_attention/README.md

configs/fast_rcnn/README.md

configs/faster_rcnn/README.md

configs/fcos/README.md

configs/foveabox/README.md

configs/fp16/README.md

configs/free_anchor/README.md