docs. Promote everything in TOC under `Starknet architecture` one level.

[stoobie]

Description of the Changes

Promote everything in TOC under Starknet architecture one level.

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Check List

• [x] Changes have been done against main branch, and PR does not conflict
• [x] PR title follows the convention: <docs/feat/fix/chore>(optional scope): <description>, e.g: fix: minor typos in code

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