Qualify getLedgers history by provider retention

[kaankacar]

Closes #52.

Applies the report, re-verified live while writing the fix: getHealth on the public testnet RPC returned oldestLedger: 3590554, latestLedger: 3711513, ledgerRetentionWindow: 120960, and getLedgers with startLedger: 2 failed with JSON-RPC -32600 — the error text itself says "must be between the oldest ledger ... for this rpc instance", which is the whole point: deep history is a property of the provider's data-lake integration, not of the method.

All four "back to genesis" references are qualified (RPC limitations bullet, the historical-data code comment, the Historical Data Access intro, and the Data Lake section), and per the issue's recommendation the code example now checks getHealth().oldestLedger before paging back.

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[kaankacar]

Verified before merging — both the failure mode and the fix's code, live.

The failure reproduces today, exactly as #52 recorded it. Raw JSON-RPC against the public SDF testnet instance:

getHealth  → oldestLedger: 3955974, latestLedger: 4076933, ledgerRetentionWindow: 120960
getLedgers(startLedger: 2) → -32600:
  "start ledger (2) must be between the oldest ledger: 3955974 and the
   latest ledger: 4076933 for this rpc instance"

The server's own error text ("for this rpc instance") is the finding in miniature — depth is a property of the instance, not the method.

The new code line was dry-run against the real SDK, not just read. With @stellar/stellar-sdk@16.2.0:

const { oldestLedger } = await rpc.getHealth();
// → 3955982 (number) — destructure works as written
rpc.getLedgers({ startLedger: 2, ... })
// → throws the same "for this rpc instance" error via the SDK path

rpc is also the variable name the rest of the file's examples already use (rpc.getAccount, rpc.sendTransaction, rpc.getLatestLedger), so the snippet drops in consistently.

Completeness: grepping the file for genesis / Infinite Scroll finds only the three qualified passages — no unqualified "back to genesis" claim survives, covering all four spots the issue flagged. The framing also now matches the official getLedgers reference word for word ("as long as the pages fall within the history retention of their corresponding RPC provider", "will return an error if startLedger is less than the oldest ledger stored in this node"). Adjacent links (Quasar) resolve; site build green. Merging.