docs: explain how wasm-opt and wasmtime are customized

Latest build toolchain docs mention *custom* `wasm-opt` and `wasmtime`. It should be explained how these are customized / differ from upstream, e.g. in [internal docs](https://github.com/Lind-Project/lind-wasm/tree/5a24e4998ab57765c8c50dc331afc47bb9e53d1d/docs/internal).

_Originally posted by @m-hemmings in https://github.com/Lind-Project/lind-wasm/pull/231#discussion_r2124364411_
            
