feat: make section link above article title clickable and configurable

The `article/section-link` hook now renders the section label as a link
instead of plain text. By default it points to the section index page
(`_index.md`).

A new `ananke.section_link` parameter (page front matter or site params)
controls the target:

- unset (default): the section index page
- "first": the first page of the section, honouring its sort order
- "<path>": a specific page, resolved relative to the section

Unresolvable values fall back to the section index. The visible label
remains the section title.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>