remove outdated op references

[roethke]

What changed? Why?
Removes references to OP which are no longer relevant post-Azul.

Node operators:

• run-a-base-node.mdx — OP_NODE_L1_ETH_RPC/OP_NODE_L1_BEACON → BASE_NODE_L1_ETH_RPC/BASE_NODE_L1_BEACON
• troubleshooting.mdx — All OP_NODE_* env vars → BASE_NODE_*, all op-node → base-consensus

Network info:

• network-fees.mdx — Replaced docs.optimism.io/stack/transactions/fees link with Base's own spec; "OP Stack's implementation of EIP-1559" → "Base's own implementation"; Jovian upgrade link docs.optimism.io/notices/upgrade-17 → /base-chain/specs/upgrades/jovian/overview
• diffs-ethereum-base.mdx — Replaced 5 docs.optimism.io links with equivalent Base docs (bridging, opcodes, address aliasing, fees, finality)
• troubleshooting-transactions.mdx — Removed broken docs.optimism.io link from DA throttling note
• block-building.mdx — Vanilla section: removed op-geth binary and dead GitHub link (op-geth no longer supported post-Azul)

API references:

• eth_getLogs.mdx, flashblocks-api-overview.mdx — "OP Stack extension" → "Base L2 extension"
• eth_getTransactionByHash.mdx — "OP Stack deposit transactions" → "Base deposit transactions"
• eth_getTransactionReceipt.mdx — All 6 "OP Stack L2 field" → "Base L2 field"

Specs:

• derivation.mdx — "(2s on Optimism)" → "(2s on Base)"; op-node → base-consensus in Engine API version section
• p2p.mdx — op-node → base-consensus in two places
• precompiles.mdx — Removed dead op-geth implementation link
• withdrawals.mdx — "Optimism L2" → "Base L2"
• azul/exec-engine.mdx — "OP Stack system contracts" → "Base system contracts"
• isthmus/derivation.mdx — mainnet.optimism.io example URL → generic Ethereum L1 RPC placeholder

Notes to reviewers

How has it been tested?
Locally

[cb-heimdall]

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[mbeutt-cb]

Most of this looks great! One thing I noticed is the block building page still references op-rbuilder instead of the base-builder: https://github.com/base/base/tree/main/crates/builder. The flashblocks architecture page does as well. It's probably worthwhile to do another sweep on keywords of everything we replaced here: https://docs.google.com/document/d/1gWb2fLfAfYJsqkOerndVs2RXOUnVChb4M2Ik2tReavc/edit?tab=t.a1c7tdkvo7f1#heading=h.ksz9k5dxohf7