docs(validity): document nonce predicate and required block expiry

[youssefea]

What changed? Why?

base/base adds a nonce validity predicate in v1.5.0 (see ValidityPredicate::Nonce in crates/execution/txpool/src/validity.rs). This PR documents it:

• Adds nonce (address, op, value) to the predicate tables on Predicates and Safety and Validity Transaction RPC.
• Adds a Nonce Example to Predicates and Safety. It covers the protocol nonce, the 0 nonce for absent accounts, and what happens when a nonce moves past an upper bound (the transaction stays pending until its block_number bound passes).
• Lists nonce on the Overview and Build a Validity Transaction pages.

It also fixes the RPC request example. Every submission must include a block_number upper bound within the 60-second window (enforced since v1.4.2, and already described under Expiry on Fees, Ordering, and Lifecycle), but the example had only a storage predicate, so the RPC would reject it as written.

Notes to reviewers

Draft until base/base 1.5.x reaches mainnet nodes. Please don't merge before then. Signed validity predicates and the fill-or-kill flag are feature-flag gated and will get separate comms, so this PR leaves them out.

An independent review by GPT-6 Astra checked the PR against base/base. Its changes are applied in the second commit:

• The block_number value in the RPC and build-guide examples is marked as a placeholder, with instructions to set it from eth_blockNumber (latest + 10).
• The nonce explanation now says it checks current protocol-nonce state only (not EIP-8130 channel nonces, deleted accounts read as 0) and does not prove that an earlier transaction succeeded.
• The expiry note no longer says a nonce-mismatched transaction stays pending until the block bound. Base checks it again when the nonce changes, and a flashblock_index bound can end it earlier.

How has it been tested?

• node scripts/lint-mdx.js on the changed files: 0 errors (7 existing highlight={} suggestions).
• npm test: pass.
• Checked predicate semantics against base/base main and the v1.5.0-rc.8 / v1.4.2 tags.

Screenshots

N/A (text and table changes only; no layout changes)

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[cb-heimdall]

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