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

website/docs/api/plugin-methods/README.mdx

@@ -13,7 +13,7 @@ Plugin APIs are shared by themes and plugins—themes are loaded just like plugi
 Every plugin is imported as a module. The module is expected to have the following members:
 
 - A **default export**: the constructor function for the plugin.
-- **Named exports**: the [static methods](./static-methods.md) called before plugins are initialized.
+- **Named exports**: the [static methods](./static-methods.mdx) called before plugins are initialized.
 
 ## Plugin constructor {#plugin-constructor}
 
@@ -35,7 +35,7 @@ type LoadContext = {
 
 ### `options` {#options}
 
-`options` are the [second optional parameter when the plugins are used](../../using-plugins.md#configuring-plugins). `options` are plugin-specific and are specified by users when they use them in `docusaurus.config.js`. If there's a [`validateOptions`](./static-methods.md#validateOptions) function exported, the `options` will be validated and normalized beforehand.
+`options` are the [second optional parameter when the plugins are used](../../using-plugins.mdx#configuring-plugins). `options` are plugin-specific and are specified by users when they use them in `docusaurus.config.js`. If there's a [`validateOptions`](./static-methods.mdx#validateOptions) function exported, the `options` will be validated and normalized beforehand.
 
 Alternatively, if a preset contains the plugin, the preset will then be in charge of passing the correct options into the plugin. It is up to the individual plugin to define what options it takes.
 

website/docs/api/plugin-methods/i18n-lifecycles.mdx

@@ -15,7 +15,7 @@ Returns translation files `{path: string, content: ChromeI18nJSON}`:
 - `path`: relative to the plugin localized folder `i18n/[locale]/[pluginName]`. Extension `.json` should be omitted to remain generic.
 - `content`: using the Chrome i18n JSON format.
 
-These files will be written by the [`write-translations` CLI](../../cli.md#docusaurus-write-translations-sitedir) to the plugin i18n subfolder, and will be read in the appropriate locale before calling [`translateContent()`](#translateContent) and [`translateThemeConfig()`](#translateThemeConfig)
+These files will be written by the [`write-translations` CLI](../../cli.mdx#docusaurus-write-translations-sitedir) to the plugin i18n subfolder, and will be read in the appropriate locale before calling [`translateContent()`](#translateContent) and [`translateThemeConfig()`](#translateThemeConfig)
 
 Example:
 

website/docs/api/plugin-methods/lifecycle-apis.mdx

@@ -111,7 +111,7 @@ export default function friendsPlugin(context, options) {
 
 This function permits one to create some global plugin data that can be read from any page, including the pages created by other plugins, and your theme layout.
 
-This data becomes accessible to your client-side/theme code through the [`useGlobalData`](../../docusaurus-core.md#useGlobalData) and [`usePluginData`](../../docusaurus-core.md#usePluginData) hooks.
+This data becomes accessible to your client-side/theme code through the [`useGlobalData`](../../docusaurus-core.mdx#useGlobalData) and [`usePluginData`](../../docusaurus-core.mdx#usePluginData) hooks.
 
 :::caution
 
@@ -405,7 +405,7 @@ Tags will be added as follows:
 
 ## `getClientModules()` {#getClientModules}
 
-Returns an array of paths to the [client modules](../../advanced/client.md#client-modules) that are to be imported into the client bundle.
+Returns an array of paths to the [client modules](../../advanced/client.mdx#client-modules) that are to be imported into the client bundle.
 
 As an example, to make your theme load a `customCss` or `customJs` file path from `options` passed in by the user: