Fix MetaMask setup guide: Update ASTR token name and network name

[armin-x86]

Summary

This PR updates the MetaMask setup guide for Astar Network to fix issues with token and network name validation.

Changes

• Changed token symbol from astr to ASTR to match MetaMask requirements. (on browser extention it is almost impossible to have astr)
• Updated network name from Astar Network Mainnet to Astar to avoid validation errors.

Issue

When users try to add the Astar network using the previous values on Chrome browser extension, they encounter errors in MetaMask:

• "This token symbol doesn't match the network name or chain ID entered."
• "According to our records, the network name may not correctly match this chain ID."

Impact

Ensures a smooth setup experience for users following the guide.

[github-actions]

Visit the preview URL for this PR (updated for commit e63bbc2):

https://astar-docs--pr728-fix-metamask-astar-t-pa2oec6s.web.app

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[Gunit2481]

LGTM