Extend FE scalability guide

[tinova]

Description

https://docs.opennebula.io/6.8/installation_and_configuration/large-scale_deployment/scalability.html#front-end-oned-scalability

also point on the relevant page with KVM maximums, which will dictate what's the max VM params will be supported

https://github.com/OpenNebula/development/tree/master/readiness/spec/scalability

We at least need:

• update the scalability numbers we have in the documentation right now
• find the scalability numbers for fireedge

Progress Status

• [ ] Code committed
• [ ] Testing - QA
• [ ] Documentation (Release notes - resolved issues, compatibility, known issues)