Feature/add inline toc
This is my idea about in-line TOC. If you think that another values of paddings, margins, colours, or whatever, will be better, feel free to mention, or improve.

First of all, thanks for the PR!
After talking a brief review, there's one catch of this approach: {{ .TableOfContent }} outputs a table of content HTML with a fixed ID: #TableOfContents. Now each page has 2 elements with the same ID, which is an invalid construction.
Now, I won't merge this PR (and no other feature PRs) soon. I'll probably write a brief article explaining the reason, but basically, due to the lack of time, and my dissatisfaction with the codebase of this theme. I would like to stop bloating this theme before I figure out a way to deal with it.
Sorry for the late reply.
https://github.com/CaiJimmy/hugo-theme-stack/discussions/630#discussioncomment-3132952 I'm afraid this PR won't be part of the theme, due to the reason explained in post above.
Thanks you for the contribution again!