[doc] added readme for documentation (#2935)

docs/README.md

+# 📕 Documentation
+
+## 🔗 Table of Contents
+
+- [📕 Documentation](#-documentation)
+  - [🔗 Table of Contents](#-table-of-contents)
+  - [📝 Overview](#-overview)
+  - [🗺 Module Structure](#-module-structure)
+  - [🧱 Our Documentation System](#-our-documentation-system)
+  - [🎊 Contribution](#-contribution)
+    - [🖊 Adding a New Documentation](#-adding-a-new-documentation)
+    - [🧹 Doc Testing](#-doc-testing)
+    - [💉 Auto Documentation](#-auto-documentation)
+
+## 📝 Overview
+
+We evaluated various existing solutions for documentation in the community and discussed their advantages and disadvangtes in the [issue #2651](https://github.com/hpcaitech/ColossalAI/issues/2651). Therefore, we propose to build a more modern and robust documentation system by integrating the Sphinx [autodoc](https://www.sphinx-doc.org/en/master/usage/extensions/autodoc.html) function and the [Docusaurus](https://docusaurus.io/) framework.
+
+## 🗺 Module Structure
+
+```text
+- docs
+    - source
+        - en
+        - zh-Hans
+    - sidebars.json
+    - versions.json
+    - requirements-doc-test.txt
+```
+
+The documentation module structure is shown above:
+1. source: This folder contains the multi-language documentation files.
+2. `sidebars.json`: The `sidebars.json` defines the table of content for the tutorials. You need to update this file when a new doc is added/deleted.
+3. `versions.json`: The `versions.json` in the **main branch** in the **latest commit** will be used to control the versions to be displayed on our website
+
+## 🧱 Our Documentation System
+
+We believe that there are several advantages from the existing system can be combined for simplicity, usability and maintainability:1
+1. Support Markdown](https://www.markdownguide.org/), we belive is a more popular language for writing documentations comapred to [RST](https://docutils.sourceforge.io/rst.html).
+2. Support Autodoc, which can automatically generate documentation from the docstrings in the source code provided by [Sphinx](https://www.sphinx-doc.org/en/master/).
+3. Support elegant and modern UI, which is provided by [Docusaurus](https://docusaurus.io/).
+4. Support MDX for more flexible and powerful documentation, which is provided by [Docusaurus](https://docusaurus.io/).
+5. Support hosting blogs/project home page/other pages besides the documentation, which is provided by [Docusaurus](https://docusaurus.io/).
+
+Therefore, we have built the [ColossalAI-Documentation](https://github.com/hpcaitech/ColossalAI-Documentation) repository to integrate the features above.
+
+## 🎊 Contribution
+
+You can contribute to the documentation by directly set up a Pull Request towards the `docs/source` folder. There are several guidelines for documentation contribution.
+
+1. The documentation is written in Markdown. You can refer to the [Markdown Guide](https://www.markdownguide.org/) for the syntax.
+2. You must ensure that the documentation exists for all languages. You can refer to the [Adding a New Documentation](#-adding-a-new-documentation) for more details.
+3. You must provide a test command for your documentation, please see [Doc Testing](#-doc-testing) for more details.
+4. You can embed your docstring in your markdown, please see [Auto Documentation](#-auto-documentation) for more details.
+
+### 🖊 Adding a New Documentation
+
+You can add a Markdown file to the `docs/source` folder`. You need to ensure that multi-language is supported in your PR.
+Let's assume that you want to add a file called `your_doc.md`, your file structure will look like this.
+
+```text
+- docs
+  - source
+    - en
+        - your_doc.md  # written in English
+    - zh-Hans
+        - your_doc.md  # written in Chinese
+  - sidebars.json  # add your documentation file name here
+```
+
+Meanwhile, you need to ensure the `sidebars.json` is updated such that it contains your documentation file. Our CI will check whether a documentation exists for all languages and can be used to build the website successfully.
+
+### 🧹 Doc Testing
+
+Every documentation is tested to ensure it works well. You need to add the following line to the top of your file and replace `$command` with the actual command. Do note that the markdown will be converted into a Python file. Assuming you have a `demo.md` file, the test file generated will be `demo.py`. Therefore, you should use `demo.py` in your command, e.g. `python demo.py`.
+
+```markdown
+<!-- doc-test-command: $command  -->
+```
+
+Meanwhile, only code labelled as a Python code block will be considered for testing.
+
+```markdown
+    ```python
+    print("hello world")
+    ```
+```
+
+Lastly, if you want to skip some code, you just need to add the following annotations to tell `docer` to discard the wrapped code for testing.
+
+```markdown
+<!--- doc-test-ignore-start -->
+
+    ```python
+    print("hello world")
+    ```
+
+<!--- doc-test-ignore-end -->
+```
+
+If you have any dependency required, please add it to `requriements-doc-test.txt`.
+
+
+### 💉 Auto Documentation
+
+Lastly, you may want to include the API documentation for a class/function in your documentation for reference.
+We support `autodoc` to extract the docstring and transform it into a Web element for elegant display.
+You just need to add `{{ autodoc:<mod-name> }}` in your markdown as a single line. An example is given below and you can see the outcome in [this PR](https://github.com/hpcaitech/ColossalAI-Documentation/pull/175).
+
+```markdown
+{{ autodoc:colossalai.amp.apex_amp.convert_to_apex_amp }}
+```