[doc] migrate the markdown files (#2652)

docs/source/en/get_started/reading_roadmap.md

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+# Reading Roadmap
+
+Colossal-AI provides a collection of parallel training components for you. We aim to support you with your development
+of distributed deep learning models just like how you write single-GPU deep learning models. ColossalAI provides easy-to-use
+APIs to help you kickstart your training process. To better how ColossalAI works, we recommend you to read this documentation
+in the following order.
+
+- If you are not familiar with distributed system or have never used Colossal-AI, you should first jump into the `Concepts`
+section to get a sense of what we are trying to achieve. This section can provide you with some background knowledge on
+distributed training as well.
+- Next, you can follow the `basics` tutorials. This section will cover the details about how to use Colossal-AI.
+- Afterwards, you can try out the features provided in Colossal-AI by reading `features` section. We will provide a codebase for each tutorial. These tutorials will cover the
+basic usage of Colossal-AI to realize simple functions such as data parallel and mixed precision training.
+- Lastly, if you wish to apply more complicated techniques such as how to run hybrid parallel on GPT-3,  the
+`advanced tutorials` section is the place to go!
+
+**We always welcome suggestions and discussions from the community, and we would be more than willing to help you if you
+encounter any issue. You can raise an [issue](https://github.com/hpcaitech/ColossalAI/issues) here or create a discussion
+topic in the [forum](https://github.com/hpcaitech/ColossalAI/discussions).**