Docs: Add plugin examples folder (#542)

Co-authored-by: Giuseppe Guerra <giuseppe.guerra@grafana.com>

docusaurus/docs/plugin-examples/_category_.json

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+{
+  "position": 55,
+  "label": "Plugin examples",
+  "collapsible": true,
+  "collapsed": true
+}

docusaurus/docs/plugin-examples/index.md

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+---
+id: plugin-examples
+title: Grafana plugin examples
+description: Sample plugins on GitHub for reference when building your own plugins.
+keywords:
+  - grafana
+  - plugins
+  - plugin
+  - samples
+  - examples
+---
+
+Sample plugins give you insights into the structure, features, and implementation details of [several types](../introduction/plugin-types-usage.md) of Grafana plugins. These are real-world code examples that you can study and even reuse in your projects.
+
+The following lists describe some of our popular plugins. For a complete list, see the [README](https://github.com/grafana/grafana-plugin-examples/tree/main#readme).
+
+## Panel plugins
+
+- [**panel-basic:**](https://github.com/grafana/grafana-plugin-examples/tree/main/examples/panel-basic) This is a good starting point for creating a simple panel plugin to visualize data, navigate dashboards, or control devices.
+- [**panel-frame-select:**](https://github.com/grafana/grafana-plugin-examples/tree/main/examples/panel-frame-select) This example shows how to update panel options with values from a data query response.
+
+## Data source plugins
+
+- [**datasource-basic:**](https://github.com/grafana/grafana-plugin-examples/tree/main/examples/datasource-basic) This app shows how to integrate a new data source including in-house metrics applications into a data source plugin.
+- [**datasource-http-backend:**](https://github.com/grafana/grafana-plugin-examples/tree/main/examples/datasource-http-backend) This app demonstrates how to query data from HTTP-based APIs, where the HTTP calls happen on the backend. Supports alerting.
+- [**datasource-logs:**](https://github.com/grafana/grafana-plugin-examples/tree/main/examples/datasource-logs) This app shows how to create a data source plugin with logging features.
+- [**datasource-streaming:**](https://github.com/grafana/grafana-plugin-examples/tree/main/examples/datasource-streaming-websocket) Update visualizations in real time using RxJS and WebSockets.
+
+## App plugins
+
+- [**app-basic:**](https://github.com/grafana/grafana-plugin-examples/blob/main/examples/app-basic/) This app plugin shows how to add fundamental features such as navigation, routing, styling, and configuration.
+- [**app-with-backend:**](https://github.com/grafana/grafana-plugin-examples/tree/main/examples/app-with-backend) This app plugin adds support for a backend including handling incoming HTTP requests.
+- [**app-with-dashboards:**](https://github.com/grafana/grafana-plugin-examples/tree/main/examples/app-with-dashboards) This example shows how to include pre-built dashboards with your app plugin.
+- [**app-with-extension-point:**](https://github.com/grafana/grafana-plugin-examples/tree/main/examples/app-with-extension-point) This example shows how to add a plugin extension point that can be extended by other plugins.
+- [**app-with-scenes:**](https://github.com/grafana/grafana-plugin-examples/tree/main/examples/app-with-scenes) Build a basic app plugin using the @grafana/scenes framework. This example shows examples of custom pages, nested data sources, and panel plugins.
+
+Refer to the Grafana plugin examples [README](https://github.com/grafana/grafana-plugin-examples/tree/main#readme) for more information on usage, testing, and API compatibility.