[DOCS] Assign users to cases

[lcawl]

See #134160

The documentation implications are as follows:

• Update screenshots and instructions in Stack Management > Cases docs [DOCS] Assign users to Stack Management cases #142925
• Update screenshots and instructions in Observability > Cases docs [DOCS] Assign users to Observability cases observability-docs#2267
• Update screenshots and instructions in Security > Cases docs [DOCS] Assign users to Security cases security-docs#2568
• Update case API docs [DOCS] Add assignees to case APIs #143610
• Optional: Update the automated case screenshots to have assignees
• Add release highlight [DOCS] Adds content to highlights docs #142419

[elasticmachine]

Pinging @elastic/response-ops (Team:ResponseOps)

[elasticmachine]

Pinging @elastic/response-ops-cases (Feature:Cases)