Update media manager documentation

[mintlify]

Updated the media manager documentation to reflect the streamlined gallery-only interface. Removed references to tree view, folders, and drag-and-drop organization features that were removed in PR #5618.

Changes:

• Updated sidebar label from "Images & Media" to "Assets"
• Changed upload instructions to reference the Upload button instead of the Add dropdown
• Removed "Organize assets" section (folders, move, rename functionality)
• Simplified "Manage assets" section to focus on delete operations
• Removed folder organization from best practices

Generated from fix: more editor nits @dks333

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Note

Refreshes editor/media.mdx to match the simplified media manager.

• Update sidebar entry to Assets and change upload flow to the Upload button
• Remove folder-based organization instructions (folders, move via drag-and-drop, rename)
• Add simplified delete flow (right-click → Delete) under a new "Manage assets" section
• Trim best practices to remove folder organization guidance

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