feat(docs): nest PR gatekeeper under Automations in the docs sidebar

[sre-helmcode]

What

Owner feedback: the PR gatekeeper guide (#175) showed at the same level as Automations. It now sits below Automations as its sub-item, in EN and ES.

Generic mechanism: optional parent frontmatter field (the parent's entry id, same value in both languages).

• Sidebar: a child renders as an indented nested <ul> right under its parent, inside the parent's group, never twice. Always visible (no toggle): one fewer click and focus stop; screen readers announce it as a level-2 list.
• Active state: the open page keeps aria-current="page" + the filled highlight; its parent gets is-trail (accent text, no fill) and the sub-list guide line turns accent.
• Breadcrumb: Docs / Automations / PR gatekeeper (URLs stay flat, /docs/pr-gatekeeper).
• Prev/next follow the sidebar's visual order (flattenDocsNav), not the raw order.
• pr-gatekeeper order 22 -> 21 and webhooks 21 -> 22 so the flat order (/api/docs manifest) matches the sidebar. Orders stay contiguous.
• Mobile: the drawer is the same sidebar component, so it nests identically. DocsTopBar lists no pages, nothing to change there.
• Robustness: unknown parent, self-parent and cycles fall back to top level (a typo costs the nesting, never the page); a child cannot open a group on its own.

Also fixes a pre-existing bug found by the render test: /es/docs (Introduccion) was marked active on every Spanish page because only /docs was excluded from prefix matching.

Files

• src/lib/docsNav.ts: nesting in groupDocsNav, flattenDocsNav, isActiveDocTrail, docsNavAncestors, docsNavItemFromEntry/docSlug (shared by layout and tests), ES index fix
• src/components/docs/DocsSidebarNav.astro, DocsNavList.astro (recursive via Astro.self)
• src/layouts/Docs.astro, src/styles/docs-shell.css, src/content.config.ts
• vitest.config.ts split into unit + render projects; vitest.render.config.ts compiles .astro for the Container API (configFile: false: the Cloudflare adapter's Vite plugin rejects vitest's SSR env)

Tests

• Unit (docsNav.test.ts): nesting, ordering of siblings, no duplicates, parent's group wins, orphan/self/cycle, multi-level, visual order for prev/next, trail, ancestors, /es/docs not active everywhere
• Render (docsNavNesting.render.test.ts, Astro Container API, real EN+ES frontmatter): PR gatekeeper in a docs-nav-sub list right under Automations, listed once, Webhooks back at top level after the block, aria-current only on the child + is-trail on Automations, no trail elsewhere
• Frontmatter guards: parent exists, not self, same group, one level, children right after parent in flat order, EN/ES parity
• npm test: 79 files / 2033 tests green; npx astro check: 0 errors; npm run build: OK

Visual check (agent-browser, local astro dev) at 1440x900 and 390x844 with the drawer open, EN and ES, plus keyboard Tab from Automations to the child (focus ring visible).

Deploy notes

No env vars, no migrations. No VERSION file in this repo; deploy on merge per the existing workflow.

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