Update docs to refer to flutter 3.24.3 (vs. 3.22.1), add Rust toolchain 1.71.0, mention secp256k1 scripts (if needed), change some ordering and grammar, etc.

Update docs to refer to flutter 3.24.3 (vs. 3.22.1), add Rust toolchain 1.71.0, mention secp256k1 scripts (if needed), change some ordering and grammar, etc. #1523