docs: Add example "Run parallel crawlers" (#1623)

### Description

- Add example "Run parallel crawlers".

Author: Mantisus

docs/examples/code_examples/run_parallel_crawlers.py

@@ -0,0 +1,94 @@
+import asyncio
+
+from crawlee import ConcurrencySettings
+from crawlee.crawlers import (
+    ParselCrawler,
+    ParselCrawlingContext,
+    PlaywrightCrawler,
+    PlaywrightCrawlingContext,
+)
+from crawlee.sessions import SessionPool
+from crawlee.storages import RequestQueue
+
+
+async def main() -> None:
+    # Open request queues for both crawlers with different aliases
+    playwright_rq = await RequestQueue.open(alias='playwright-requests')
+    parsel_rq = await RequestQueue.open(alias='parsel-requests')
+
+    # Use a shared session pool between both crawlers
+    async with SessionPool() as session_pool:
+        playwright_crawler = PlaywrightCrawler(
+            # Set the request queue for Playwright crawler
+            request_manager=playwright_rq,
+            session_pool=session_pool,
+            # Configure concurrency settings for Playwright crawler
+            concurrency_settings=ConcurrencySettings(
+                max_concurrency=5, desired_concurrency=5
+            ),
+            # Set `keep_alive`` so that the crawler does not stop working when there are
+            # no requests in the queue.
+            keep_alive=True,
+        )
+
+        parsel_crawler = ParselCrawler(
+            # Set the request queue for Parsel crawler
+            request_manager=parsel_rq,
+            session_pool=session_pool,
+            # Configure concurrency settings for Parsel crawler
+            concurrency_settings=ConcurrencySettings(
+                max_concurrency=10, desired_concurrency=10
+            ),
+            # Set maximum requests per crawl for Parsel crawler
+            max_requests_per_crawl=50,
+        )
+
+        @playwright_crawler.router.default_handler
+        async def handle_playwright(context: PlaywrightCrawlingContext) -> None:
+            context.log.info(f'Playwright Processing {context.request.url}...')
+
+            title = await context.page.title()
+            # Push the extracted data to the dataset for Playwright crawler
+            await context.push_data(
+                {'title': title, 'url': context.request.url, 'source': 'playwright'},
+                dataset_name='playwright-data',
+            )
+
+        @parsel_crawler.router.default_handler
+        async def handle_parsel(context: ParselCrawlingContext) -> None:
+            context.log.info(f'Parsel Processing {context.request.url}...')
+
+            title = context.parsed_content.css('title::text').get()
+            # Push the extracted data to the dataset for Parsel crawler
+            await context.push_data(
+                {'title': title, 'url': context.request.url, 'source': 'parsel'},
+                dataset_name='parsel-data',
+            )
+
+            # Enqueue links to the Playwright request queue for blog pages
+            await context.enqueue_links(
+                selector='a[href*="/blog/"]', rq_alias='playwright-requests'
+            )
+            # Enqueue other links to the Parsel request queue
+            await context.enqueue_links(selector='a:not([href*="/blog/"])')
+
+        # Start the Playwright crawler in the background
+        background_crawler_task = asyncio.create_task(playwright_crawler.run([]))
+
+        # Run the Parsel crawler with the initial URL and wait for it to finish
+        await parsel_crawler.run(['https://crawlee.dev/blog'])
+
+        # Wait for the Playwright crawler to finish processing all requests
+        while not await playwright_rq.is_empty():
+            playwright_crawler.log.info('Waiting for Playwright crawler to finish...')
+            await asyncio.sleep(5)
+
+        # Stop the Playwright crawler after all requests are processed
+        playwright_crawler.stop()
+
+        # Wait for the background Playwright crawler task to complete
+        await background_crawler_task
+
+
+if __name__ == '__main__':
+    asyncio.run(main())

docs/examples/run_parallel_crawlers.mdx

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+---
+id: run-parallel-crawlers
+title: Run parallel crawlers
+---
+
+import ApiLink from '@site/src/components/ApiLink';
+import RunnableCodeBlock from '@site/src/components/RunnableCodeBlock';
+
+import RunParallelCrawlersExample from '!!raw-loader!roa-loader!./code_examples/run_parallel_crawlers.py';
+
+This example demonstrates how to run two parallel crawlers where one crawler processes links discovered by another crawler.
+
+In some situations, you may need different approaches for scraping data from a website. For example, you might use <ApiLink to="class/PlaywrightCrawler">`PlaywrightCrawler`</ApiLink> for navigating JavaScript-heavy pages and a faster, more lightweight <ApiLink to="class/ParselCrawler">`ParselCrawler`</ApiLink> for processing static pages. One way to solve this is to use <ApiLink to="class/AdaptivePlaywrightCrawler">`AdaptivePlaywrightCrawler`</ApiLink>, see the [Adaptive Playwright crawler example](./adaptive-playwright-crawler) to learn more.
+
+The code below demonstrates an alternative approach using two separate crawlers. Links are passed between crawlers via <ApiLink to="class/RequestQueue">`RequestQueue`</ApiLink> aliases. The `keep_alive` option allows the Playwright crawler to run in the background and wait for incoming links without stopping when its queue is empty. You can also use different storage clients for each crawler without losing the ability to pass links between queues. Learn more about available storage clients in this [guide](/python/docs/guides/storage-clients).
+
+<RunnableCodeBlock className="language-python" language="python">
+    {RunParallelCrawlersExample}
+</RunnableCodeBlock>