docs: add example plugin implementation (#735)

* docs: add example plugin implementation

Author: shadowusr

.eslintignore

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 tmp
 **/playwright-report
 **/html-report/*
+examples/**

examples/random-number-plugin-example/README.md

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+# HTML Reporter Plugin Example
+
+## What it does
+
+Adds a collapsible "Random Number" section to test results that fetches a random number from the server when expanded.
+
+![Screenshot](plugin-screenshot.png)
+
+## Project Structure
+
+```
+├── ui/                  # Frontend plugin (React + Redux)
+├── server/              # Express middleware
+├── preset-for-config/   # Config preset factory
+└── package.json
+```
+
+## Building
+
+```bash
+npm install
+npm run build
+```
+
+## Usage
+
+1. Install the plugin in your project (or link it locally using `npm link random-number-plugin-example` for development)
+
+2. Update your `testplane.config.ts`:
+
+```ts
+import randomNumberPluginUI from 'random-number-plugin-example';
+
+export default {
+    // ... other config
+    plugins: {
+        'html-reporter/testplane': {
+            enabled: true,
+            // Enable plugins and add plugin preset to html-reporter config
+            pluginsEnabled: true,
+            plugins: randomNumberPluginUI(),
+        },
+    },
+};
+```
+
+3. Run your tests with html-reporter enabled — the plugin section will appear in test results.