docs: add in CodeGroup and CodeBlock usage docs (#2621)

Author: d-pollard

packages/docs/docs/theme/default-theme-config.md

@@ -557,6 +557,90 @@ layout: SpecialLayout
 
 This will render `.vuepress/components/SpecialLayout.vue` for the given page.
 
+## Code Groups and Code Blocks <Badge text="1.6.0+" />
+The default theme comes with custom components `<code-group />` and `<code-block>` that can help you to better group
+separate code paths that have the same result.
+
+**Example**
+
+Yarn and npm installation instructions
+
+**Input**
+````md
+<code-group>
+<code-block title="YARN">
+```bash
+yarn create vuepress-site [optionalDirectoryName]
+```
+</code-block>
+
+<code-block title="NPM">
+```bash
+npx create-vuepress-site [optionalDirectoryName]
+```
+</code-block>
+</code-group>
+````
+
+**Output**
+
+<code-group>
+<code-block title="YARN">
+```bash
+yarn create vuepress-site [optionalDirectoryName]
+```
+</code-block>
+
+<code-block title="NPM">
+```bash
+npx create-vuepress-site [optionalDirectoryName]
+```
+</code-block>
+</code-group>
+
+::: tip
+Each `<code-block />` must have a line break after it to render properly
+:::
+
+### Default Code Block
+
+By default, `<code-group />` will use the first `<code-block />` as the active initial code block.
+
+To manually set the active `<code-block />`, just add the `active` prop to the `<code-block />`:
+
+**Input**
+````md
+<code-group>
+<code-block title="YARN">
+```bash
+yarn create vuepress-site [optionalDirectoryName]
+```
+</code-block>
+
+<code-block title="NPM" active>
+```bash
+npx create-vuepress-site [optionalDirectoryName]
+```
+</code-block>
+</code-group>
+````
+
+**Output**
+
+<code-group>
+<code-block title="YARN">
+```bash
+yarn create vuepress-site [optionalDirectoryName]
+```
+</code-block>
+
+<code-block title="NPM" active>
+```bash
+npx create-vuepress-site [optionalDirectoryName]
+```
+</code-block>
+</code-group>
+
 ## Ejecting
 
 You can copy the default theme source code into `.vuepress/theme` to fully customize the theme using the `vuepress eject [targetDir]` command. If you didn’t install VuePress globally, run `./node_modules/.bin/vuepress eject`.