Anomaly detection documentation reorganization

[lcawl]

Goal

Related to #1725

The goal of this issue is to track the effort of creating a new, more user-friendly structure for the anomaly detection documentation.

Reorganized TOC

Anomaly detection

• Detecting anomalies ... #1747, #1750, #1753
  • Split into concrete tasks [DOCS] Split anomaly detection into multiple pages #1943
• Advanced concepts #1731
• API quick reference
• Resources #1731, #74802

Link fixes:

• [DOCS] Update forecasting links in ML APIs elasticsearch#74942
• [DOCS] Fix links to anomaly detection overview elasticsearch#74943
• [DOCS] Fixes links to anomaly detection overview kibana#104350
• [DOCS] Fixes links to anomaly detection overview security-docs#782
• [DOCS] Add redirects for broken links #1751
• [DOCS] Adds anchor for anomaly detection job links #1752
• [ML] Updates URLs in documentation link service kibana#105011
• [DOCS] Fixes links to machine learning concepts elasticsearch#75194
• [DOCS] Fixes link to ML custom rules security-docs#801

[szabosteve]

I close it in favor of https://github.com/elastic/ml-team/issues/838.