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

packages/docusaurus-utils-validation/src/index.ts

@@ -0,0 +1,22 @@
+/**
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+import * as Joi from '@hapi/joi';
+
+const MarkdownPluginsSchema = Joi.array()
+  .items(
+    Joi.array()
+      // TODO, this allows [config,fn] too?
+      .items(Joi.function().required(), Joi.object().required())
+      .length(2),
+    Joi.function(),
+  )
+  .default([]);
+
+export const RemarkPluginsSchema = MarkdownPluginsSchema;
+export const RehypePluginsSchema = MarkdownPluginsSchema;
+
+export const AdmonitionsSchema = Joi.object().default({});