Remove/make the warning box in 'Header' less prominent

[avivace]

The warning box about forgetting to reserve space in the header doesn’t add new or actionable information. The surrounding text already explains the mistake and explicitly notes that rgbfix (≥ 0.5.1) detects and warns about it automatically. There's no case in which the reader is not already using 0.5.1 since we explicitly target higher versions.

rgbfix already raises the warning, highlighting it again in a warning box:

• overemphasizes a handled case,
• interrupts the reading flow,
• increases cognitive load for beginners

This would work better as a regular paragraph or a brief inline note, rather than a prominent warning box