Add English version of `datasets.md`, the Chinese version is in
https://github.com/open-mmlab/mmsegmentation/pull/2387

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Co-authored-by: MeowZheng <meowzheng@outlook.com>

* draft

* update loss

* update

* add runner

* add steps

* update

* [WIP][Doc] Add Data Structures and Elements

* fix

* add

* fix

* add chinses doc

* refactor

* fix

* fix typo

* fix

* fix

* fix typo

* Update docs/en/advanced_guides/structures.md

* Update docs/en/advanced_guides/structures.md

Co-authored-by: Miao Zheng <76149310+MeowZheng@users.noreply.github.com>

* introduction

* add evaluator doc and fix typo

* fix ut

* add test dataflow

* add runner reference

* add dataloader config

* fix typo

* minor change

* develop segmentor and remove custom optim

* update segmentor example

* add data preprocessor

* refine intro

* [Docs] Add Overview

* comments

* refine

* refine

* [Fix] README for mmseg 1.x

* typo

* link and refine

* [WIP] Refine documentation

* get started done

* config refine

* train_test

* refine user guides

* add contribution

* add contribution

* refine visualization

* advanced tutorial

* advanced guides

* tricks

* refine zh doc

* refactor changelog

* transform tutorial

* fix

* [Doc] MMSeg 1.x Documentation

* revise index