[Docs migration] Cleans up getting started files

[nastasha-solomon]

• Removes raw migrated Serverless files (content and images) 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:

• Get started
  • Elastic Security requirements
  • Ingest data into Elastic Security
    • Enable threat intelligence integrations
  • Spaces and Elastic Security
  • Configure advanced settings