docs(site): add cellagdiffexp docs, demote tucca-rna-seq to legacy

[benjibromberg]

Adds a documentation section for the new cellagdiffexp Nextflow pipeline and makes it the featured pipeline on the site, demoting the older Snakemake tucca-rna-seq to a kept-but-legacy status (its docs and URLs stay intact).

What changed

New docs section site/src/content/docs/cellagdiffexp/ (4 pages, ported from the pipeline repo's canonical docs, adapted to Starlight):

• introduction.mdx — what it is (downstream of nf-core/rnaseq, matrix-in), feature overview, successor note.
• usage.mdx — inputs, contrasts (simple + advanced/interaction/DREAM), annotation + enrichment params, SPIA (native + KO routes), DE options, running examples (test/test_chicken/test_pig).
• output.mdx — every output directory.
• architecture.mdx — the Lean B owned-vs-reused boundary, data-flow diagram, ADR summaries, per-phase history, verification strategy.

Navigation (cellagdiffexp is now primary; tucca-rna-seq kept as legacy):

• Sidebar (astro.config.mjs): new cellagdiffexp group; tucca-rna-seq relabeled (legacy) and collapsed.
• Navbar (SiteTitle.astro): the Docs dropdown now points at cellagdiffexp (with a legacy link kept); cellagdiffexp added to the Projects dropdown and the mobile menu.
• Landing page (index.mdx): new cellagdiffexp project card; tucca-rna-seq card marked legacy.
• tucca-rna-seq/introduction.mdx: the roadmap note strengthened into a "Frozen: superseded by cellagdiffexp" banner.

Notes

• Strategy: deprecate + demote (not delete). All 18 tucca-rna-seq pages and their public URLs are preserved (no 404s, useful Tufts HPC guides kept); it is just no longer the featured pipeline.
• Build verified locally: pnpm build clean on node 22 (all 30 pages, including the 4 new ones; sidebar/nav slugs resolve). No em dashes, no AI-tell vocabulary.
• Deploys to the live site on merge to main, so left unmerged for your review.

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