[Docs migration] Cleans up detection and alerts files

[nastasha-solomon]

Addresses part of #559 by doing the following:

• Removes raw migrated Serverless files that are no longer being used.
• Adds anchors where missing and updates anchors that start with an underscore.
• Makes minor inline adjustments to the files that were labeled as "lift-and-shift" where appropriate.

Affected pages:

• About building block rules
• About detection rules
• Add detection alerts to cases
• Create a detection rule
• Detections requirements
• Install and manage Elastic prebuilt rules
• Launch Timeline from investigation guides
• Manage detection alerts
• Manage detection rules
• MITRE ATT&CK® coverage
• Using logsdb index mode with Elastic Security
• View detection alert details
• Visualize detection alerts
• Alert suppression
• Reduce alerts and notifications
• Query alert indices
• Tune detection rules

[benironside]

Few tiny optional suggestions, otherwise looks perfect!