Update MathCAT logo and alt text

[ways2read]

Summary

• Replace the MathCAT logo with the new artwork and add a regular-resolution source PNG
• Add descriptive alt text explaining that the cat's tail forms the C in MathCAT
• Constrain the logo display size so it sits consistently next to the page titles

Test plan

• Check the README and docs pages render the new logo at a reasonable size
• Confirm the alt text reads correctly with a screen reader
• Spot-check English and Russian docs that include the logo

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