chore: update README files

apps/emqx_conf/README.md

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+# Configuration Management
+
+This application provides configuration management capabilities for EMQX.
+
+This includes, during compilation:
+- Read all configuration schemas and generate the following files:
+  * `config-en.md`: documentation for all configuration options.
+  * `schema-en.json`: JSON description of all configuration schema options.
+  * `emqx.conf.example`: an example of a complete configuration file.
+
+At runtime, it provides:
+- Cluster configuration synchronization capability.
+  Responsible for synchronizing hot-update configurations from the HTTP API to the entire cluster
+  and ensuring consistency.
+
+In addition, this application manages system-level configurations such as `cluster`, `node`, `log`.

apps/emqx_plugins/README.md

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+# Plugins Management
+
+This application provides the feature for users to upload and install custom, Erlang-based plugins.
+
+More introduction about [Plugins](https://www.emqx.io/docs/en/v5.0/extensions/plugins.html#develop-emqx-plugins)
+
+See HTTP API to learn how to [Install/Uninstall a Plugin](https://www.emqx.io/docs/en/v5.0/admin/api-docs.html#tag/Plugins)
+
+## Plugin Template
+
+We provide a [plugin template](https://github.com/emqx/emqx-plugin-template) that
+you can use to learn how to write and package custom plugins.