docs(fees): document fee recipients, L1 fee pass-through and gas payment

[youssefea]

What changed? Why?

• Network Fees: adds sections on how the L1 fee is passed through, where each fee component goes (no fees are burned; each accrues in a fee vault predeploy), how to estimate a transaction's total cost and how it changes with demand, and options for paying gas without ETH (paymaster sponsorship, ERC-20 gas payment, x402 facilitator settlement, EIP-8130 payers), including what CDP Paymaster error -32002 means.
• B20: notes that B20 does not implement ERC-3009 and links to the gas payment options.

Notes to reviewers

None.

How has it been tested?

node scripts/lint-mdx.js on changed files, node scripts/validate-docs-structure.js, npm test; external links checked.

Screenshots

Not captured; text-only additions to two existing pages.

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[cb-heimdall]

🟡 Heimdall Review Status

Requirement	Status	More Info
Reviews	🟡 0/1	Denominator calculation Show calculation 1 if user is bot 0 1 if user is external 0 2 if repo is sensitive 0 From .codeflow.yml 1 Additional review requirements Show calculation Max 0 0 From CODEOWNERS 0 Global minimum 0 Max 1 1 1 if commit is unverified 0 Sum 1

[linear]

DEVREL-817