docs: sync upgrade, RPC, and node operator pages after Cobalt

[youssefea]

What changed? Why?

• Snapshots: replaces the "mid-August" V1 decommission warning with the actual October 5, 2026 cutoff (linked to the status notice) and points the Reth V2 page to the snapshot migration paths.
• Public RPC: documents that mainnet.base.org no longer serves the debug, trace, and txpool namespaces and has a lower read rate limit (Connect to Base, RPC overview, Run a Node).
• Transaction Finality: aligns the Flashblock reorg figure with the < 0.1% target stated in Throughput and Limits and the Flashblocks reference, and states the Mainnet proposal interval.
• Proposer spec: adds the current Mainnet BLOCK_INTERVAL (600) and INTERMEDIATE_BLOCK_INTERVAL (30), with the cast commands to read them.
• Run a Node: explains --rpc.eth-proof-window (reth default and maximum) for eth_getProof without the proofs ExEx.
• Azul proofs: notes that Beryl reduced the single-proof window from 7 to 5 days.
• B20 changelog: moves the seize surface and burnBlocked deprecation out of the Beryl entry, since they shipped with Cobalt.

Notes to reviewers

The interval values come from AggregateVerifier (game type 621) on Ethereum Mainnet; the eth-proof-window values come from reth's rpc-server-types constants.

How has it been tested?

node scripts/lint-mdx.js on changed files (0 errors), node scripts/validate-docs-structure.js, npm test; external links checked.

Screenshots

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[linear]

DEVREL-816