[Doc] MMSeg 1.x Documentation (#1924)

docs/en/advanced_guides/add_dataset.md

docs/en/advanced_guides/add_modules.md

docs/en/advanced_guides/customize_runtime.md

docs/en/advanced_guides/data_flow.md

docs/en/advanced_guides/datasets.md

docs/en/advanced_guides/engine.md

docs/en/advanced_guides/evaluation.md

docs/en/advanced_guides/index.rst

docs/en/advanced_guides/models.md

docs/en/advanced_guides/structures.md

docs/en/advanced_guides/training_tricks.md

docs/en/advanced_guides/transforms.md

docs/en/index.rst

docs/en/migration.md

docs/en/notes/changelog.md

docs/en/notes/faq.md

docs/en/tutorials/index.rst

docs/en/user_guides/1_config.md

docs/en/user_guides/2_dataset_prepare.md

docs/en/user_guides/3_inference.md