[Doc] Improve the documentation to reflect the max amount of vCPU supported

[xmarcalx]

Feature Request

Improve the documentation and give better clarity of how many vCPU are supported into Firecracker.

Describe the desired solution

Documentation should state the max number of vCPU supported

Describe possible alternatives

N/A

Additional context

As result of the survey, some of our customers were not aware that Firecracker support already up to 32 vCPU. As part of this issue we would like to improve our documentation to make such feature more explicit.

Checks

• [X] Have you searched the Firecracker Issues database for similar requests?
• [X] Have you read all the existing relevant Firecracker documentation?
• [X] Have you read and understood Firecracker's core tenets?