🎨 Palette: Add aria-hidden to decorative GitBook icons

[ErisonBarros]

💡 What:
Added aria-hidden="true" to FontAwesome <i> tags in guitbook/aulas/aula-2.md. Also recorded a new UX/a11y journal entry for raw HTML GitBook icons.

🎯 Why:
To prevent screen readers from announcing confusing textual fallbacks (e.g., :envelope:) embedded alongside decorative icons within GitBook markdown content.

📸 Before/After:
N/A - Visually identical.

♿ Accessibility:
Improved screen reader experience by hiding decorative icon tags that contained redundant textual fallbacks meant for markdown parsing.

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PR created automatically by Jules for task 5394577482302012533 started by @ErisonBarros

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