chore: add apps gallery sample plugin

samples/sample-apps-gallery-item-plugin/README.md

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+# Sample Apps Gallery Item Plugin
+
+## What is it?
+
+The Sample Apps Gallery Item Plugin serves as a demonstration of how developers can create their own custom plugins, in this case it adds an entry in the apps gallery.
+
+![Gif of plugin demo](./public/assets/plugin.gif)
+
+## Building the Plugin
+
+To build the plugin for production use, follow these steps:
+
+```bash
+cd $HOME/src/plugin-apps-gallery-item
+npm ci
+npm run build-bundle
+```
+
+The above command will generate the `dist` folder, containing the bundled JavaScript file named `AppsGalleryItemPlugin.js`. This file can be hosted on any HTTPS server along with its `manifest.json`.
+
+If you install the Plugin separated to the manifest, remember to change the `javascriptEntrypointUrl` in the `manifest.json` to the correct endpoint.
+
+To use the plugin in BigBlueButton, send this parameter along in create call:
+
+```
+pluginManifests=[{"url":"<your-domain>/path/to/manifest.json"}]
+```
+
+Or additionally, you can add this same configuration in the `.properties` file from `bbb-web` in `/usr/share/bbb-web/WEB-INF/classes/bigbluebutton.properties`

samples/sample-apps-gallery-item-plugin/manifest.json

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+{
+  "requiredSdkVersion": "~0.0.70",
+  "name": "SampleAppsGalleryItemPlugin",
+  "javascriptEntrypointUrl": "SampleAppsGalleryItemPlugin.js",
+  "localesBaseUrl": "https://cdn.dominio.com/pluginabc/"
+}