fix: Correct URL matcher fallback behavior and improve memory monitoring

Fix critical issue where unmatched URLs incorrectly used the first config instead of failing safely. Also clarify that configs without url_matcher match ALL URLs by design, and improve memory usage monitoring.

Bug fixes:
- Change select_config() to return None when no config matches instead of using first config
- Add proper error handling in dispatchers when no config matches a URL
- Return failed CrawlResult with "No matching configuration found" error message
- Fix is_match() to return True when url_matcher is None (matches all URLs)
- Import and use get_true_memory_usage_percent() for more accurate memory monitoring

Behavior clarification:
- CrawlerRunConfig with url_matcher=None matches ALL URLs (not nothing)
- This is the intended behavior for default/fallback configurations
- Enables clean pattern: specific configs first, default config last

Documentation updates:
- Clarify that configs without url_matcher match everything
- Explain "No matching configuration found" error when no default config
- Add examples showing proper default config usage
- Update all relevant docs: multi-url-crawling.md, arun_many.md, parameters.md
- Simplify API config examples by removing extraction_strategy

Demo and test updates:
- Update demo_multi_config_clean.py with commented default config to show behavior
- Change example URL to w3schools.com to demonstrate no-match scenario
- Uncomment all test URLs in test_multi_config.py for comprehensive testing

Breaking changes: None - this restores the intended behavior

This ensures URLs only get processed with appropriate configs, preventing
issues like HTML pages being processed with PDF extraction strategies.

docs/md_v2/api/arun_many.md

@@ -131,7 +131,7 @@ github_config = CrawlerRunConfig(
 # API endpoints - JSON extraction
 api_config = CrawlerRunConfig(
     url_matcher=lambda url: 'api' in url or url.endswith('.json'),
-    extraction_strategy=JsonCssExtractionStrategy({"data": "body"})
+    # Custome settings for JSON extraction
 )
 
 # Default fallback config
@@ -160,6 +160,7 @@ results = await crawler.arun_many(
 - Each URL is processed by the same or separate sessions, depending on the dispatcher’s strategy.
 - `dispatch_result` in each `CrawlResult` (if using concurrency) can hold memory and timing info.  
 - If you need to handle authentication or session IDs, pass them in each individual task or within your run config.
+- **Important**: Always include a default config (without `url_matcher`) as the last item if you want to handle all URLs. Otherwise, unmatched URLs will fail.
 
 ### Return Value