Alerting docs: Update and restructure `Introduction/Alert rule evaluation`

[ppcano]

• Update Alert rules evaluation (current docs).

  • Add a mermaid diagram, UI screenshot, and table.
  • Update content for clarity.

• Relocate to Introduction > Alert rules evaluation.

• Also, Introduction > Alert rules evaluation > State and health of alert rules.

Next, we could improve the State and health of alert rules page - I think we can do it in a separate PR.