feat: render Article and Talk pages with Astro

[jpoissonnet]

Preview

https://feat-astro-pages.dsvmt7xpjktgx.amplifyapp.com

Stacked on #497. Production stays on Jekyll.

Summary

• Base.astro ports the Jekyll head, navbar and footer: the dark-mode script, Matomo (skipped on previews via SITE_PREVIEW, like JEKYLL_ENV=preview), the SEO, Open Graph, Twitter and JSON-LD tags, the RSS link, and <html lang>.
• src/styles/site.scss overrides the three font paths and imports _sass/type-on-strap.scss unchanged. Its Sass deprecation warnings are silenced, and Lightning CSS strips the old IE *zoom hacks it cannot parse.
• [...slug].astro builds every Article and Talk at its permalink. build.format: 'file' writes the same files as Jekyll (a/b and a/b.html both become a/b.html). Talks keep the YouTube, Slideshare and event blocks of the video and conference layouts, and previous/next navigation stays per collection.
• [...redirect].ts writes each redirect_from page at its exact path, with the same HTML as jekyll-redirect-from.
• remark-kramdown.mjs handles the two kramdown features still in use: {:target="_blank"} after links (56, in 5 Articles) and {:toc} (3 Articles). Heading ids use GitHub slugs, like Jekyll's GFM parser.
• Markdown goes through @astrojs/markdown-remark (unified), since Astro 7's default processor doesn't run remark plugins.
• Future-dated entries are hidden in production and shown on previews, like Jekyll's --future.
• _articles/2024-11-12-golab-florence.md: a blank line before a --- that CommonMark read as a heading underline, where kramdown read a horizontal rule.

Known differences from production, on purpose or for later:

• Code blocks use Astro's default Shiki theme without the Rouge styling until PR 4.
• Inline author: objects (old Tumblr posts) now show the author name, where Jekyll rendered empty spans.
• The footer says "Powered by Astro".

Test plan

• Local build: all 337 permalinks and 18 redirects exist at the path Jekyll writes.
• Local build against production, all 337 pages: same heading ids, same images, no new broken #anchor, no literal {:...}.
• Screenshots on the Amplify preview: the first screen of /data/2017/10/23/genesis-of-m6-datalake.html and the Talk page are pixel-identical to production.
• Preview: /data/2017/10/23/genesis-of-m6-datalake.html
• Preview: Tumblr-era /post/23664141031/lancement-du-blog-technique-dm6web (with and without .html)
• Preview: a Talk, /2013/10/distribuez-votre-video-partout-avec-3-euros-en-poche-et-devenez-millionaire-ou-presque.html
• Preview: a redirect, /hsdo/
• Preview: navbar icons and fonts load, dark mode toggles