Outdated documentation on developing changes to SpiderMonkey

https://github.com/bytecodealliance/StarlingMonkey/blob/main/docs/src/developer/spidermonkey.md

The information in the file above is outdated as it mentions `gecko-dev` branch that is now superseded by https://github.com/bytecodealliance/firefox/tree/wasi-ff140. I think the other instructions in this file are not up to date as well.