docs(fees): explain options for paying gas in tokens

[youssefea]

What changed? Why?

Adds a "Paying Gas in Tokens" section to docs/specifications/transactions/network-fees.mdx. It explains that Base charges fees in ETH, that B20 doesn't add a protocol-level fee token, that CDP Paymaster's ERC-20 gas payment currently lists only USDC on Base, and that Base Transactions (EIP-8430) token gas payments are planned for Everest, with activation dates still TBD.

This closes Base Knowledge Base gap gap-20260930-erc-20-b20-gas-payment-partner: "Can a partner's own ERC-20 or B20 token be used to pay transaction gas on Base (native fee-token support in B20, or an ERC-20 paymaster), and if so what is the supported mechanism and any restrictions?"

• Gaps log: https://bdocs.cbhq.net/d/base-knowledge-gaps-log
• Linear: DEVREL-859

Notes to reviewers

Primary sources for each fact:

• CDP Paymaster ERC-20 gas payment lists only USDC on Base ("Additional tokens coming soon"): https://docs.cdp.coinbase.com/paymaster/guides/erc20-gas-payments
• B20 is a superset of ERC-20, and neither page describes a fee-token feature: docs/upgrades/beryl/b20.mdx (ERC-20 Compatibility)
• Base Transactions token gas payments (the sponsor pays ETH and is reimbursed in a token in phase 0): docs/specifications/base-transactions.mdx (Token Gas Payments, and the Warning on Everest activation)
• Everest Sepolia and Mainnet dates are TBD: docs/upgrades/everest/overview.mdx

Owner confirmation needed before merge:

• CDP Paymaster: confirm that USDC is still the only accepted token on Base, and say whether projects can request enablement for other tokens. The page tells readers not to assume any other ERC-20 or B20 token is accepted.
• B20 and Base protocol owners: confirm that there is no protocol-level fee token for B20, and confirm the "planned, TBD" framing for Base Transactions token gas payments.

This replaces the gas-payment part of #2090 for this gap. That PR's "Paying Gas Without ETH" section predates the current Base Transactions (EIP-8430) and Everest pages, and it described EIP-8130 on vibenet. The other parts of #2090 (fee recipients, L1 fee pass-through, cost estimation, and the B20 ERC-3009 note) are out of scope here.

How has it been tested?

• node scripts/lint-mdx.js --files-from=<changed files>: passed, with 0 errors and 0 warnings.
• node scripts/validate-docs-structure.js: it reports 2 failures that already exist on master and that this PR doesn't touch (redirect /upgrades/denim/eip-8130 -> /specifications/native-account-abstraction).
• Internal links (/upgrades/beryl/b20, /specifications/base-transactions#token-gas-payments, /upgrades/everest/overview) are present in docs/docs.json, and the anchor heading exists.
• External links return 200 with curl -sIL.

Screenshots

N/A (text-only change)

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[cb-heimdall]

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[linear]

DEVREL-859