Structural search improvements and documentation

Created by: rvantonder

These are work items improve the quality of structural search in 3.11:

• Get match patterns to hit the index where spaces in structural search patterns can match newlines
  • PR #6851
  • e2e integration test
• Address zip issues (Assert_failure zip.ml:212:4) #7134
• Address worker management
  • Orphaned process management (partial solutions: #7177 #7162)
  • Gracefully handle worker OOM #7187
• Honor lang parameter scope #7179
• Multiline highlighting #6899, #7018
• Add experimental flag (default ON, with possibility to disable) #7194
• where rule filter #7180
• Update patterntype URL param, "literal" messages, and routing #7267
• Metrics for failures #7249
• Documentation #7324

Triaged items for 3.12 are tracked in #7234 (closed).