docs(node-operators): add Run a Local Devnet guide

[roethke]

What changed? Why?

Adds Specifications → Node Operators → Run a Local Devnet, a one-page guide for teams that want a local Base stack. Until now, the devnet was documented only in base/base's etc/docker/README.md, which targets base/base contributors.

The page covers:

• Prerequisites: Docker with Compose v2 + Buildx, just ≥ 1.51.0, Foundry, jq, and a base/base clone
• Start: just devnet up-single vs just devnet up (3-node HA conductor cluster), then just devnet status
• Test transaction: cast send using the prefunded Anvil accounts from etc/docker/devnet-env (same flow as base/base's local-devnet-transaction-inclusion verification skill)
• Endpoints: builder 7545, RPC nodes 8545 and 8645, L1 4545, Grafana 3000; chain IDs 84538453 / 1337
• Hardforks: Cobalt at block 22 and Denim at block 25 by default; everest / zenith modes, with a warning that op-batcher stalls on EIP-8130 blocks; L2_BASE_*_BLOCK overrides
• Common commands and Learn More links to the devnet README, the Justfile, docs/guides/TESTING.md, and docs/guides/STANDALONE_PROVING.md

Notes to reviewers

• Placement: I put it in Node Operators, after Run a Node, because the IA guidelines scope that group to "content for people operating infrastructure". Build on Base → Overview (next to Test on Vibenet) is the alternative if we want app developers to find it. The page links to Vibenet for teams that don't need local infra.
• Scope left out on purpose: the Anvil + Nitro proving variant, profiling, tx-observability, the batcher shadow topology, and upgrade-signal commands. All are linked through the README.
• Please verify (base/base owners): that up-single is the right default to recommend over up for external teams, and that the Everest/Zenith modes are OK to mention publicly. Every value on the page was copied from base/base main (etc/docker/Justfile, devnet-env, docker-compose.yml) on 2026-09-29 and rechecked against main (577c04d19) on 2026-10-01.
• Chain ID: the local L2 uses 84538453, the same as hosted Vibenet.
• Please verify: whether just devnet flashblocks still streams after the Denim cutover (block 25), since the README says builder selection changes at Denim.

How has it been tested?

• node scripts/lint-mdx.js on the new page: 0 errors, 0 warnings
• node scripts/validate-docs-structure.js: passes (nav, orphans, links)
• Commands, ports, and fork blocks were checked against base/base main. I did not run a devnet locally for this PR.

Screenshots

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[cb-heimdall]

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[roethke]

Moving back to draft: waiting for updates to the devnet before merging.