Commands in the WASM guide should use pinned versions of rustc and emsdk

[chitoyuu]

It appears that the versions of LLVM used in rustc and emscripten have drifted out of sync again. Although the guide does specify a pair of known good versions of the tools, the commands simply use nightly and tot instead, and as a result some linking issues may arise when commands are blindly copied.

There are several steps we can take here. Firstly, the commands should be modified to use pinned versions. Longer term, we should see if there's a consistent way to tell the compatible version of emscripten from any given rustc version, or if we have to stick to manually finding known good pairs for the foreseeable future.

We should also see if #87 is still viable and if it can be merged into the current guide.

[chitoyuu]

User necrashter on Discord found a more recent pair of working toolchains:

rustc 1.69.0-nightly (ef982929c 2023-01-27) (currently latest nightly) and Emscripten 3.1.21 (f9e81472f1ce541eddece1d27c3632e148e9031a) combination also works

Thanks!