Add necessity to list all steps of a spec algorithm to style guide

Per https://github.com/servo/servo/issues/37022 we should be adding steps to relevant algorithms. This can be added to the style guide with some examples. Things to look out for:

1. Are all steps listed, those unimplemented have a `// TODO` on the next line
2. Steps ending with `.`
3. If there is spec text that doesn't link to a particular step, is there a link where it can be found