docs(website): use .mdx extension for every docs (#8490)

website/versioned_docs/version-2.0.1/api/themes/overview.mdx

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+---
+sidebar_position: 0
+id: themes-overview
+sidebar_label: Themes overview
+slug: /api/themes
+---
+
+# Docusaurus themes
+
+We provide official Docusaurus themes.
+
+## Main themes {#main-themes}
+
+The main themes implement the user interface for the [docs](../plugins/plugin-content-docs.mdx), [blog](../plugins/plugin-content-blog.mdx) and [pages](../plugins/plugin-content-pages.mdx) plugins.
+
+- [@docusaurus/theme-classic](./theme-classic.mdx)
+- 🚧 other themes are planned
+
+:::caution
+
+The goal is to have all themes share the exact same features, user-experience and configuration.
+
+Only the UI design and underlying styling framework should change, and you should be able to change theme easily.
+
+We are not there yet: only the classic theme is production ready.
+
+:::
+
+## Enhancement themes {#enhancement-themes}
+
+These themes will enhance the existing main themes with additional user-interface related features.
+
+- [@docusaurus/theme-live-codeblock](./theme-live-codeblock.mdx)
+- [@docusaurus/theme-search-algolia](./theme-search-algolia.mdx)