fix(website): show per-page last updated dates and update deps (#1940)

- Add a prebuild script that fetches full git history when the clone is
  shallow, so Starlight's "Last updated" date reflects each page's last
  commit instead of the latest commit in the repo
- Update @astrojs/starlight to 0.42.3, astro to 7.3.4, sharp to 0.35.4
  and starlight-llms-txt to 0.12.0

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

Author: coliff