Add table of contents sidebar and linkable headers

[dannon]

Summary

• Adds automatic table of contents (TOC) sidebar for article pages, matching the old Gridsome behavior
• Adds clickable anchor links to all headings for easy section linking

Features

Table of Contents:

• Displays on the right sidebar (desktop) or as a collapsible section (mobile)
• Shows h2-h4 headings with proper indentation
• Only appears when page has 2+ headings
• Controlled via autotoc frontmatter field (opt-in, matching old behavior)

Linkable Headers:

• All headings now have anchor links that appear on hover
• Clicking copies the section URL to the address bar
• Uses # symbol, styled to be unobtrusive

Technical Changes

• Added rehype-slug and rehype-autolink-headings plugins
• Created TableOfContents.astro component
• Updated ArticleLayout.astro and event pages to support TOC
• Page templates now extract and pass headings from rendered content

Fixes #3569

[bgruening]

Imho, this does not look good. All headers have an indentation that looks off. Unfortunately also the append looks not good as it is on the far right of the site.

Playing around with it, I think append could work. I would prefer that over prepand

[bgruening]

I rebased, some test where failing for me locally, so I fixed that as well.

For me the prepand option does not look good, see my comment above.
The TOC is great!

[bgruening]

Cool!

[dannon]

Thanks for the suggestions and merge!