docs: remove retired legacy RPCs - #65

Docs-only cleanup that removes the retired sei2_* namespace and obsolete sei_* methods; the removal is complete and internally consistent (no dangling references, explorer data matches the reference table, eth_getVMError correctly covers the dropped sei_getVMError). No blockers, but the rewrite drops version markers and per-method migration paths, hand-edits two verbatim server strings that need confirming against sei-chain #3927, and leaves some local inconsistencies in the seid CLI page.