docs(fees): update EIP-1559 parameters and log the March/May 2026 changes

[kriss39]

What changed? Why?

The EIP-1559 parameters documented on network-fees and throughput-and-limits (elasticity 6, denominator 125 on both networks) have not matched the chain since March. Reading them out of the post-Holocene block header extraData (0x01 | denominator (u32) | elasticity (u32)), the history is:

Network	Block	Date (UTC)	Change
Base Mainnet	43841214	2026-03-25 20:49	denominator 125 → 100, elasticity 6 → 5, gas limit 375M → 400M
Base Sepolia	39348279	2026-03-25 18:54	denominator 125 → 100, elasticity 6 → 5, gas limit 375M → 400M
Base Sepolia	41252949	2026-05-08 21:03	elasticity 5 → 15, gas limit 400M → 1.2G

SystemConfig.eip1559Elasticity() / eip1559Denominator() / gasLimit() on L1 return the same values today (mainnet 5 / 100 / 400000000, Sepolia 15 / 100 / 1200000000).

This PR:

• updates the elasticity/denominator prose, worked example and tables on network-fees.mdx (mainnet 5/100 → 4% max change per block; Sepolia 15/100 → 14%)
• updates the fee parameter table on throughput-and-limits.mdx
• adds the missing rows to the configuration changelog for both networks (the 1559 change and the gas limit changes; the last logged 1559 entry was the February bump to 125)

Notes to reviewers

The "18 blocks / 36 seconds to double" example still holds for mainnet: (5-1)/100 = 4% per block and 1.04^18 ≈ 2.03, same as before.

To reproduce: cast block 43841213 -f extraData --rpc-url https://mainnet.base.org vs cast block 43841214 …, or cast call 0x73a79Fab69143498Ed3712e519A88a918e1f4072 'eip1559Elasticity()(uint32)' --rpc-url <l1>.

How has it been tested?

node scripts/lint-mdx.js on the three files and node scripts/validate-docs-structure.js pass.

Screenshots

N/A (table values and two sentences change; no layout change)

[cb-heimdall]

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