Remove obsolete documentation screenshot assets

[robinebers]

TL;DR

Remove the unused docs/assets screenshot directory and its obsolete Markdown references.

What was happening

• Five committed PNG screenshots added repository weight and unrelated binary churn.
• Two menu-bar documentation screenshots were the directory's only remaining references.

What this changes

• Delete all five PNG files and remove docs/assets entirely.
• Remove the two obsolete screenshot blocks from the existing menu-bar documentation.
• Keep the separate README screenshot under the top-level assets directory unchanged.

Tests

• Verified there are no remaining docs/assets or removed screenshot references.

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Note

Low Risk
Documentation-only edit with no code, security, or runtime impact.

Overview
Removes the two screenshot blocks from the Hiding usage while screen sharing section of docs/menu-bar.md (idle strip vs. wordmark while sharing). The prose for that setting is unchanged.

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