Update the SIEM section in the Kibana doc

[benskelker]

We need to update:

• Screenshots - if you want, use the ones in this PR
• Add endgame-* as a default SIEM index pattern
• Add very short sentence on Elastic Endpoint Security (Endgame):
  • Elastic Endpoint Sensor Management Platform
    The Sensor Management Platform (SMP) ships all relevant host and network data directly to the SIEM app.
• Check I haven't forgotten anything (probably have)

Feel free to change and improve the current doc.

Thanks,