chore(deps): upgrade to Astro 7 and Starlight 0.41 (#339)

## Summary

Replaces #337 (dependabot), which could not merge as-is: Astro 7 /
Starlight 0.41 need repo-side changes. Dependabot's bump commit is
preserved here, with the required fixes on top.

- `astro` 6.3.2 → 7.1.6, `@astrojs/starlight` 0.38.1 → 0.41.6
(cherry-picked from #337)
- Wrap all autogenerate sidebar configs in `items` arrays (Starlight
0.39 removed labelled autogenerated groups)
- Normalise the sidebar shape in `scripts/postprocess-motoko.mjs` so
synced Motoko pages keep working
- Fix `deriveSections()` in `plugins/astro-agent-docs.mjs` so llms.txt
keeps its section labels, and make the same class of failure fail the
build in future
- Migrate `markdown.remarkPlugins` / `rehypePlugins` to `processor:
unified({ ... })`
- Align `validate.yml` on Node 22

## Why the extra changes

**Sidebar (build blocker).** Starlight 0.39 requires the autogenerate
config to sit inside an `items` array:

```diff
  {
    label: "Backends",
    collapsed: true,
-   autogenerate: { directory: "guides/backends" },
+   items: [{ autogenerate: { directory: "guides/backends", collapsed: true } }],
  }
```

`collapsed` is set on the autogenerate config too, because autogenerated
subgroups no longer inherit the parent's collapsed state as of 0.39. No
autogenerate directory currently has subdirectories, so this is a no-op
today and keeps behaviour correct if one is added.

**Motoko sidebar (would regress on the next sync).**
`sidebar-motoko.mjs` is generated from `doc/site/sidebar.mjs` in the
pinned `.sources/motoko` submodule, which still uses the pre-0.39 shape.
Hand-editing the generated file alone would be undone by the next Motoko
sync, breaking the build again. Instead `transformEntry()` now
normalises both shapes, so the output is valid whichever shape upstream
emits and no coordination with `caffeinelabs/motoko` is required.
Verified: both shapes produce byte-identical output, and regeneration is
idempotent.

**llms.txt (silent break, now guarded).** `deriveSections()` read the
section label off the autogenerate node. Once nested, that node is
unlabelled and the label lives on the parent group, so all 15
autogenerated sections rendered as `## undefined` in llms.txt — with a
clean build log and a zero exit code. Beyond fixing the derivation, the
derived sections are now validated at module load and the build throws
with the offending directory and the function to update. Verified by
reintroducing the original bug: the build fails immediately instead of
shipping. This guards the whole class of failure, since these labels are
inferred from the sidebar's shape and future Starlight majors can change
it again.

**Markdown processor.** Astro 7 defaults to the Sätteri processor and
deprecates the top-level plugin arrays (the build warned about it).
`processor: unified({ ... })` opts back into the remark/rehype pipeline
the 5 remark and 2 rehype plugins need. `@astrojs/markdown-remark` is
declared as a direct dependency instead of relying on Astro's transitive
copy; only the root dependency entry is added to the lockfile.

**Node 22 in `validate.yml`.** Astro 7 requires `^20.19.0 || >=22.12.0`.
The previous `'20'` pin resolved to a satisfying 20.x and that job does
not run Astro, so this aligns it with the other five workflows rather
than fixing a break.

## Verification

Built against a `main` baseline and compared output:

- 209 pages build clean, no errors, no deprecation warnings
- Sidebar identical: 38 groups, same labels, same collapsed states, 196
nav links
- `llms.txt`, `llms-full.txt`, `feed.xml` and all 209 markdown endpoints
byte-identical
- Rendered article content identical on 204 of 208 pages; the 4
differences are 3 smart-quote corrections (`”Canister is not ready”` →
`“Canister is not ready”`) and one whitespace change between flex items
in a Starlight LinkCard on the homepage, which does not render
- Confirmed on the deployed preview: the submodule-sourced Motoko pages
that use `file=` includes render in full, and `llms.txt` is
byte-identical to the local build
- Build time 33s → 28s

## Notes

- Starlight 0.41 drops support for Chromium < 111 and Safari < 16.4.
- Upstream `caffeinelabs/motoko` pins Starlight ^0.41.3 in
`doc/site/package.json` while its `sidebar.mjs` still uses the pre-0.39
shape, and no workflow there builds `doc/site`, so their own preview
site hits this same error unnoticed. Worth a separate upstream PR; this
branch does not depend on it.

---------

Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: marc0olo

.github/workflows/validate.yml

@@ -15,7 +15,7 @@ jobs:
 
       - uses: actions/setup-node@v4
         with:
-          node-version: '20'
+          node-version: '22'
           cache: 'npm'
 
       - run: npm ci

astro.config.mjs

@@ -1,6 +1,7 @@
 // @ts-check
 import { defineConfig } from "astro/config";
 import starlight from "@astrojs/starlight";
+import { unified } from "@astrojs/markdown-remark";
 import rehypeRewriteLinks from "./plugins/rehype-rewrite-links.mjs";
 import rehypeExternalLinks from "./plugins/rehype-external-links.mjs";
 import remarkIcpCliVersion from "./plugins/remark-icp-cli-version.mjs";
@@ -16,10 +17,15 @@ import { TITLE, DESCRIPTION, PUBLISHER, OG_ALT } from "./src/branding.mjs";
 export default defineConfig({
   site: "https://docs.internetcomputer.org",
   markdown: {
+    // Astro 7 defaults to the Sätteri processor; `unified()` opts back into the
+    // remark/rehype pipeline these plugins need. Top-level `markdown.remarkPlugins`
+    // and `markdown.rehypePlugins` are deprecated in favour of this.
     // Rehype plugins work with Starlight (remark plugins don't — Starlight overrides them).
     // See: https://github.com/dfinity/icp-cli/issues/423
-    rehypePlugins: [rehypeRewriteLinks, rehypeExternalLinks],
-    remarkPlugins: [remarkHeadingId, remarkSnippet, remarkIcpCliVersion, remarkPlantUML, remarkIncludeFile],
+    processor: unified({
+      rehypePlugins: [rehypeRewriteLinks, rehypeExternalLinks],
+      remarkPlugins: [remarkHeadingId, remarkSnippet, remarkIcpCliVersion, remarkPlantUML, remarkIncludeFile],
+    }),
   },
   integrations: [
     starlight({