feat(v2): markdown pages (#3158)

* markdown pages POC

* add remark admonition, mdx provider, yarn2npm...

* pluginContentPages md/mdx tests

* pluginContentPages md/mdx tests

* add relative file path test link to showcase link problem

* fix Markdown pages issues after merge

* fix broken links found in markdown pages

* fix tests

* factorize common validation in @docusaurus/utils-validation

* add some documentation

* add using plugins doc

* minor md pages fixes

website/src/pages/examples/markdownPageExample.md

@@ -0,0 +1,34 @@
+---
+title: Markdown Page example title
+description: Markdown Page example description
+---
+
+# Markdown page
+
+This is a page generated from markdown to illustrate the markdown page feature.
+
+It supports all the regular MDX features, as you can see:
+
+:::info
+
+Useful information.
+
+:::
+
+```jsx live
+function Button() {
+  return (
+    <button type="button" onClick={() => alert('hey')}>
+      Click me!
+    </button>
+  );
+}
+```
+
+![](../../../static/img/docusaurus.png)
+
+import Tabs from '@theme/Tabs';
+
+import TabItem from '@theme/TabItem';
+
+<Tabs defaultValue="apple" values={[ {label: 'Apple', value: 'apple'}, {label: 'Orange', value: 'orange'}, {label: 'Banana', value: 'banana'} ]}><TabItem value="apple">This is an apple 🍎</TabItem><TabItem value="orange">This is an orange 🍊</TabItem><TabItem value="banana">This is a banana 🍌</TabItem></Tabs>