fix(docs): replace external OP Stack fee link with Base documentation reference

[schoolkamsergj]

Summary

Fixes the stale external link in the network fees documentation raised in #1395 (Point 1).

The network-fees.mdx page linked to https://docs.optimism.io/stack/transactions/fees (OP Stack developer docs) for fee mechanics. Since Base now maintains its own documentation and stack, this external reference is misleading.

Change

Replaced the external OP Stack link with a reference to the Base network fees overview page.

Before:

For additional details about fee calculation on Base, please refer to the
[op-stack developer documentation](https://docs.optimism.io/stack/transactions/fees).

After:

For additional details about fee calculation on Base, please refer to the
[Base network fees overview](/base-chain/network-information/network-fees).

Why

• Base runs on its own stack since the post-OP-Stack migration
• Linking to Optimism docs for Base fee mechanics creates confusion for developers
• The existing page itself already covers fee details — the self-reference keeps users within Base docs

Closes #1395 (Point 1)

[cb-heimdall]

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