Add a sticky left sidebar TOC that scrolls with the page

[yuccichou]

I added a sidebar with table of contents, and I tried to match the original webpage style.

I also tried to make the webpage responsive (mobile-friendly) but failed. It was much harder than I thought.

Summary by Bito

This pull request introduces a sticky left sidebar for the table of contents, enhancing navigation. It also includes CSS adjustments for improved layout responsiveness, although full mobile-friendliness remains a challenge. These changes aim to enhance user experience and accessibility.

Unit tests added: False

Estimated effort to review (1-5, lower is better): 2