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.

tests/test_multi_config.py

@@ -55,13 +55,13 @@ async def test_multi_config():
     
     # Test URLs - using real URLs that exist
     test_urls = [
-        # "https://www.w3.org/WAI/ER/tests/xhtml/testfiles/resources/pdf/dummy.pdf",  # Real PDF
-        # "https://www.bbc.com/news/articles/c5y3e3glnldo",  # News article
-        # "https://blog.python.org/",  # Blog URL  
-        # "https://api.github.com/users/github",  # GitHub API (returns JSON)
-        # "https://httpbin.org/json",  # API endpoint that returns JSON
-        # "https://www.python.org/",  # Generic HTTPS page
-        # "http://info.cern.ch/",  # HTTP (not HTTPS) page
+        "https://www.w3.org/WAI/ER/tests/xhtml/testfiles/resources/pdf/dummy.pdf",  # Real PDF
+        "https://www.bbc.com/news/articles/c5y3e3glnldo",  # News article
+        "https://blog.python.org/",  # Blog URL  
+        "https://api.github.com/users/github",  # GitHub API (returns JSON)
+        "https://httpbin.org/json",  # API endpoint that returns JSON
+        "https://www.python.org/",  # Generic HTTPS page
+        "http://info.cern.ch/",  # HTTP (not HTTPS) page
         "https://example.com/",  # → Default config
     ]