docs: add merkle tree usage documentation

[airope]

Closes #3374

Adds documentation for merkle tree usage including:

• Overview and concepts
• Creating merkle trees from data
• Generating proofs
• Hashing leaves
• Code examples with region markers

New files:

• apps/docs/src/guide/utilities/merkle-tree.md
• apps/docs/src/guide/utilities/snippets/merkle/index.ts
• Sidebar entry in .vitepress/config.ts

[cursor]

PR Summary

Low Risk
Low risk: documentation-only change adding a new guide page and snippet code, with no runtime/SDK logic modifications.

Overview
Adds a new Merkle Tree guide under Utilities, documenting how to build trees, compute roots, generate proofs, and hash leaves using @fuel-ts/merkle.

Includes a new snippets/merkle/index.ts example (region-marked for embedding) and updates the VitePress sidebar to link to /guide/utilities/merkle-tree.

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