Add table of contents and back-to-top button to blog posts

[manuelzzz]

Adds a table of contents and a back-to-top button to long-form blog posts, to make navigating them easier.

• Table of contents: adds BlogTableOfContents (packages/site_shared/lib/components/blog/blog_toc.dart), which renders an automatically generated, collapsible "On this page" nav from the post's rendered headings. It's built on jaspr_content's existing TableOfContentsExtension rather than a custom implementation, so it correctly builds anchor links relative to the current page path. Posts with fewer than 2 top-level headings don't show a TOC.
• Back-to-top button: adds BackToTopButton (packages/site_shared/lib/components/common/client/back_to_top_button.dart), a floating button that appears once the reader scrolls past the hero and smooth-scrolls back to the top on click. It reuses the site's existing in-content scroll-spy signal, so no new scroll-tracking JS was needed.
• Fix: heading anchors on flutter.dev (for example, TOC links, or #some-heading URLs) were landing behind the fixed header instead of scrolling clear of it. The existing scroll-margin-top rule in _content.scss referenced --site-header-height/--site-subheader-height, which are defined for docs.flutter.dev but were never defined for this site, silently making the rule a no-op. Defines them, aliased to the existing --ui-header-height variable.

Both features are implemented in the shared site_shared components (not sites/www or sites/docs directly) and only wired into the blog layout for now, so they're reusable for other long-form content later.

Closes #13737

[gemini-code-assist]

Code Review

This pull request introduces a table of contents component (BlogTableOfContents) and a floating "back to top" button (BackToTopButton) to the blog layout, along with their respective styling and configuration. A critical syntax error was found in the table of contents component where a leading dot was incorrectly placed before the text function call.

[abdallahshaban557]

/gcbrun

[flutter-website-bot]

Staged preview of the updated docs.flutter.dev site (updated for commit 61e7ec5):

https://flutter-docs-prod--docs-pr13870-blog-toc-and-back-to-t-c50jekze.web.app

[sfshaza2]

/gcbrun

[parlough]

Thanks for tackling these additions! I will review it as soon as I can. Hopefully today or tomorrow.

If possible, could you do two things to help the process?

1. Split this PR into two: One with the table of contents and heading fixes and another with the back-to-top button. That way they can be reviewed and landed independently.
2. Add a screenshot of the relevant feature to the PR descriptions.

[flutter-website-bot]

Staged preview of the updated flutter.dev site (updated for commit 61e7ec5):

https://flutter-dev-230821--www-pr13870-blog-toc-and-back-to-t-z03wa0gx.web.app

[manuelzzz]

Thanks for tackling these additions! I will review it as soon as I can. Hopefully today or tomorrow.

If possible, could you do two things to help the process?

1. Split this PR into two: One with the table of contents and heading fixes and another with the back-to-top button. That way they can be reviewed and landed independently.
2. Add a screenshot of the relevant feature to the PR descriptions.

I'm already working on this, in a few minutes i will open these two PR's

[manuelzzz]

I will close this PR and open the second PR as recommended.

The first one is #13881 and is already opened.