docs: common RPC errors & troubleshooting guide #810

A short docs page covering the most common RPC errors developers hit when connecting to Sentrix (wrong chain id, mismatched RPC URL, nonce/gas issues, rate limits) with the fix for each.

Good first issue — no chain internals needed, just clear writing and a few reproductions against the public RPC. Pointers: the existing RPC/network docs and the public endpoints.