Link to the spec from appropriate places

[jyasskin]

https://wicg.github.io/scheduling-apis/ from e.g.

• The About section of https://github.com/WICG/scheduling-apis
• By https://github.com/WICG/scheduling-apis#explainer-links
• https://github.com/WICG/scheduling-apis/blob/main/explainers/prioritized-post-task.md

[shaseley]

Thanks! And good timing, I'm working on an explainer cleanup pass :).