[SecuritySolutions] Update risk score docs and add links to existing workflows

[machadoum]

Overview:

As an analyst, I want a summary of the risk-scoring algorithm within my workflows to refer to during my triaging and investigation activities.

Acceptance Criteria:

• [ ] Users can view the below when risk scores are displayed:

  • An overview of the risk-scoring algorithm
  • How risk scores map to risk levels
  • Reference to expanded risk score documentation

[elasticmachine]

Pinging @elastic/security-threat-hunting (Team:Threat Hunting)