Add Table of Contents (TOC) feature

[EmbraceLife]

The Table of Contents (TOC) feature automatically generates a sidebar index for every blog post. When someone visits a blog post, the system:

1. Scans the notebook for all markdown headings (#, ##, ###, etc.) and records each heading's level, text, and a URL-friendly slug (like section-1-how-themes-work)

2. Rewrites the article by attaching a unique id attribute to every heading — so each heading becomes clickable anchor point on the page

3. Builds a sidebar with a linked list of those headings — each link points to the corresponding heading's id

4. Places them side by side — the article on the left, the TOC sidebar on the right in a flexible layout. On mobile, the sidebar hides and a floating button appears instead, opening a slide-out panel with all the section links

5. Highlights as you scroll — as you scroll through the article, whichever section is currently visible gets highlighted in the TOC sidebar (scrollspy effect)

The feature was built and refined across 5 battles:

• Sticky TOC — wrapping the sidebar in a sticky CSS container so it stays visible while scrolling
• HTMX refresh — moving the TOC inside the swap target so it updates when navigating between posts without a full page reload
• Long-line width — fixing code blocks that pushed the entire page wider than the screen with white-space: pre-wrap
• Mobile squeeze — hiding the TOC wrapper on small screens so the article uses full width
• Code block bug — switching from one-pass regex to split-process-reassemble, so Python comments inside code blocks are not mistaken for section headings

[RensDimmendaal]

Thank you @EmbraceLife I'll review it, might take a few days though!

[RensDimmendaal]

@EmbraceLife I see some code msgs have double #| export tags, also there's a rt not found code output could you clean those up please?

[RensDimmendaal]

@EmbraceLife I guess this is your custom theme? Can you remove it please?