[Request] Move blog posts' "Table Of Contents" generation into layout

Currently the Table Of Contents on each blog post is a reused snippet of code that has to be copy-pasted into each new post. It should be possible to move this into the `_layouts/post.html` template that all of the posts inherit from, so that the ToC is always generated without having to be copy-pasted. This needs to be done without adding any new Jekyll plugins which may not be available in the GitHub builder environment.