feat(v2): Improve the initial templates

packages/docusaurus-init/templates/classic/docs/create-a-doc.md

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+---
+title: Create a Document 
+---
+
+This page will help you to create your first document with Docusaurus.
+
+## Creating a simple document 
+
+Create a markdown file `docs/hello.md`.
+
+```md
+website # root directory of your site
+├── docs
+│   └── hello.md
+├── src
+│   └── pages
+├── docusaurus.config.js
+├── ...
+```
+
+At the top of the file, specify the `id` and `title` of your document. The purpose is so that Docusaurus can identify your document when generating your site. After that you can start adding your document's content.
+
+```mdx
+---
+id: hello 
+title: Hello, World! 
+---
+
+## Hello, World!
+
+This is your first document in **Docusaurus**, Congratulations!
+```
+
+:::note
+
+By default, id of the document is the `filename` of the document. So, you can omit it if your filename already resembles the id.
+
+:::
+
+
+
+After creating and saving the file, run your development server and go to `http://localhost:3000/docs/hello` in your browser.
+
+## Adding your document to the sidebar
+
+You can add `hello.md` to the sidebar by adding it to `sidebars.js`
+
+Open `sidebars.js` and add the `id` of `hello.md` which is `hello` to the items in docs sidebar.
+
+```js
+module.exports = {
+  docs: [
+    {
+      type: 'category',
+      label: 'Docusaurus Tutorial',
+      items: ['getting-started', 'markdown-features', 'create-a-doc', 'hello'],
+    },
+  ],
+};
+```
+
+Check your sidebar now, and you can access `hello.md` from the docs sidebar now!
+
+
+## Making your document as the home page
+
+If you want to make `hello.md` as your docs home page, then you can add `slug` to the header fields.
+
+```mdx
+---
+id: hello
+title: Hello, World! 
+slug: /
+---
+
+Hello, World!
+```

packages/docusaurus-init/templates/classic/docs/create-a-page.md

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+---
+title: Create a Page
+---
+
+This page will help you to create standalone pages in Docusaurus, either with React or Markdown.
+
+## Creating a React Page
+
+Create a file: `/src/pages/react.js`
+
+```jsx title="/src/pages/react.js"
+import React from "react";
+
+function HelloWorld() {
+    return (
+        <div>
+            <h1>Hello, World!</h1>
+        </div>
+    )
+}
+```
+
+Save the file, and the development server will automatically reload the changes. Open `http://localhost:3000/react`, to see the page that you just created with React. 
+
+## Creating a Markdown Page
+
+Create a file: `/src/pages/markdown.md`
+
+```mdx title="/src/pages/markdown.md"
+---
+title: Hello, World!
+description: This is a page created with Markdown
+hide_table_of_contents: true
+---
+
+# Hello, World!
+
+```
+
+Save the file, and the development server will automatically reload the changes. Open `http://localhost:3000/markdown`, to see the page that you just created with Markdown. 
+
+## Routing
+
+Any JavaScript (React) or Markdown files that you create under `/src/pages` directory will be automatically converted into a website page.
+
+Here are some examples:
+
+- `/src/pages/index.js` -> `<baseUrl>`
+- `/src/pages/foo.js` -> `<baseUrl>/foo`
+- `/src/pages/foo/bar.js` -> `<baseUrl>/foo/bar`

packages/docusaurus-init/templates/classic/docs/create-a-post.md

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+---
+title: Create a Post 
+---
+
+This page will help you on how to create blog posts in Docusaurus.
+
+## Adding Posts
+
+In order to add a post in the blog, create a file: `/blog/2021-02-28-greetings`.
+
+The format for filename of a post is `YYYY-MM-DD-post-title.md`, you have to follow this format because the post date is extracted from the file name.
+
+```md
+---
+slug: greetings
+title: Greetings! 
+author: Steven Hansel 
+author_title: Docusaurus Contributor 
+author_url: https://github.com/ShinteiMai
+author_image_url: https://avatars.githubusercontent.com/u/54180475?s=460&u=dea92f5adfe1adb82d983200553508851c3a96a0&v=4 
+tags: [greetings, docusaurus]
+---
+
+Congratulations, you have made your first post!
+
+Feel free to play around and edit this post as much you like.
+
+```
+
+The post should be generated automatically after you save the file. Open `http://localhost:3000/blog/greetings`, and you should see your created post!
+