fix(sidebar): main sidebar adjustments

[marcus-souza-azion]

What & why

The docs home sidebar was a flat list of 34 rows where every row was a one-way trip into another sidebar. Two of those bands now open in place instead. Fundamentals becomes a dropdown whose pages keep the home sidebar rather than swapping to a sidebar of their own, and Platform stops being a group label over nine product rows and becomes a single dropdown holding How Azion works — the platform explainer, moved here from Fundamentals where it sat oddly — and Core services, a nested dropdown with the nine products. The collapsed home sidebar drops from 34 rows to 26, and the reader gets one obvious place to learn what the platform is. Build, Store, Secure, Observe and Manage keep their current shape.

Both are the same mechanism: a nav tree marked "inline": true renders inside the root sidebar instead of owning one, so its rows expand in place, its pages render the home sidebar with the active row highlighted, and none of its rows show the › that means "this replaces the sidebar". The tree files still exist, so breadcrumbs, prev/next and every navcheck contract keep working untouched — no changes to lint-navcheck.ts and no exempt.json entries. The second commit adds the label the open section was missing: a section's own menu now names itself above its first rows, in the design system's group-title treatment, on the rail, in the mobile drawer and in a menu opened from a chevron.

Pages affected:

• /documentation/platform/ · /documentacao/plataforma/ — the explainer, moved here and retitled "How Azion works" / "Como a Azion funciona"
• /documentation/fundamentals/ · /documentacao/fundamentos/ — new landing pages taking over those URLs
• Links re-pointed in: use-a-template.mdx, start-with-a-template.mdx, edge-application.mdx, overview.mdx (Runtime), usar-um-template-via-rtm.mdx, comecar-com-um-template.mdx, visao-geral-runtime.mdx, plus src/data/agent-setup.json and src/i18n/en/footer.ts
• Nav model: src/nav/resolve.ts, schema.ts, root.json, trees/fundamentals.json, trees/platform.json (new)

Type of change

• 🆕 New content (feat) — the two Fundamentals landing pages, en + pt-br
• 🩹 Fix (fix) — typo, broken link, wrong information
• ♻️ Content update (docs) — rewrite, expansion, upkeep — the explainer retitled and moved
• 🌐 Translation sync (i18n)
• 🏗️ Platform / structure (refactor / chore)

Author checklist

• PR title follows type(scope): summary (see GOVERNANCE.md §4)
• Frontmatter complete: title, description, meta_tags, namespace, permalink, last_reviewed — all four pages carry the first five; last_reviewed is on zero pages corpus-wide
• No legacy "edge-" product names in the copy
• How-to/tutorial content includes at least one runnable, copy-paste-tested code block — no how-to or tutorial in this PR
• Screenshots (if any) have alt text and follow image standards — none added
• Internal links are relative and resolve locally — every re-pointed target checked on the dev server
• If any permalink changed or page moved: redirect added in this PR — permalinks did change, and no redirect applies: the old URL is reoccupied by the new landing page in the same commit, so nothing dangles, and a redirect there would make that page unreachable
• i18n: pt-br updated in this PR or follow-up i18n issue created:
• I ran pnpm build:local (build + frontmatter check) without errors