docs: Document Cairo runtime semantics

[lisenokdonbassenok]

Add a proper runtime reference page for Cairo instead of the previous placeholder.

• Describe the high-level execution flow from Sierra to CASM and the Cairo VM.
• Explain how SierraCasmRunner prepares functions, arguments, gas, and result extraction.
• Clarify the relationship between the runtime, the VM, and the relocatable memory model.
• Document gas handling, execution resources, and StarknetExecutionResources.
• Outline how StarknetState, the hint processor, and syscalls interact with the simulated environment.
• Detail how success, panic, gas failures, and VM errors are surfaced through RunResult and RunnerError.
• Add implementation references and an explicit “out of scope” section for VM internals and prover details.

[reviewable-StarkWare]

This change is 

[orizi]

Reviewable status: 0 of 1 files reviewed, 2 unresolved discussions (waiting on @lisenokdonbassenok)

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docs/reference/src/components/cairo/modules/language_semantics/pages/runtime.adoc line 5 at r1 (raw file):

== Overview

This page describes the runtime execution model of Cairo programs as implemented in this

everything about the VM should probably be mostly an external link.

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docs/reference/src/components/cairo/modules/language_semantics/pages/runtime.adoc line 7 at r1 (raw file):

This page describes the runtime execution model of Cairo programs as implemented in this
repository. It explains how compiled programs are run on the Cairo VM, how gas and execution
resources are tracked, and how Starknet-specific system calls interact with the simulated

the description of all the structs is mostly useless for any one reading this module.
something much more concise is required.

[orizi]

No response.

Closing.