TRE Instruction for simpler multicalls

[Voxelot]

[Link to related issue(s) here, if any]

• Add Transfer assets back to the script context from the contract context fuel-vm#965

[Short description of the changes.]
This pull request adds documentation for a new instruction to the Fuel VM instruction set, specifically describing the TRE operation for transferring coins to an external context. The most important changes are grouped below:

Instruction Set Documentation Updates

• Added a new entry for the TRE instruction ("Transfer coins to external context") to the instruction index list in src/fuel-vm/instruction-set.md.
• Provided a detailed specification for the TRE instruction, including its description, operation, syntax, encoding, effects, usage notes, panic conditions, receipt structure, and state changes. This explains how assets are transferred from an internal contract context to the external script context and the requirements for successful execution.

This functionality enables simpler patterns for chaining multi-calls across different applications with scripts. If TRE is used to return the funds to the user after each operation such as "swap" or "borrow", applications won't need to have any special logic to manage transfers between each other (see workarounds here).

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• I have reviewed the code myself

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• Rust SDK
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