feat(v2): generalize usage of _ prefix convention to exclude content files/folders (#5173)

* create a swizzleWarning partial for shared text

* Generalize usage of _ prefix convention to exclude content files/folders

* add api doc

* MDX loader should not expect metadata/frontmatter on MDX partial files

website/docs/api/plugins/plugin-content-docs.md

@@ -69,6 +69,15 @@ module.exports = {
          */
         routeBasePath: 'docs',
         include: ['**/*.md', '**/*.mdx'], // Extensions to include.
+        /**
+         * No route will be created for matching files
+         */
+        exclude: [
+          '**/_*.{js,jsx,ts,tsx,md,mdx}',
+          '**/_*/**',
+          '**/*.test.{js,jsx,ts,tsx}',
+          '**/__tests__/**',
+        ],
         /**
          * Path to sidebar configuration for showing a list of markdown pages.
          */