docs: remove stale OP Stack fee reference

[dieutx]

Summary

• remove the outdated OP Stack fee documentation link from the Base network fees page
• replace it with a self-contained pointer to the relevant Base-specific sections already on the page

Problem

Issue #1395 notes that /base-chain/network-information/network-fees still points readers to OP Stack developer docs even though Base now runs on the unified base/base stack.

Solution

This change removes the external OP Stack reference and directs readers to the Base fee mechanics sections already documented on the page:

• minimum base fee
• EIP-1559 fee parameters
• querying the L1 fee

Verification

• confirmed the stale OP Stack link existed in docs/base-chain/network-information/network-fees.mdx
• updated the page copy to remove the obsolete reference without changing page structure or navigation

Partially addresses #1395

[cb-heimdall]

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