feat(nav): restructure top navigation and sidebars

The old nav used bare lowercase slugs and leaned on filesystem order for
every sidebar.

- Replace the three existing nav entries with six labelled ones, keyed
  through i18n.json so both locales resolve to their own text from
  identical _nav.json files.
- Add _meta.json for blogs and opensource-projects, grouping entries by
  topic instead of leaving them alphabetical.
- Rename the Blogs and Notifications pages so their titles match the nav
  labels (Insights and News).
- Point the notification page's account-settings link at GitHub, as the
  Gitea instance it referenced is retired.

docs/en-gb/opensource-projects/_meta.json

@@ -0,0 +1,14 @@
+[
+  "index",
+  { "type": "section-header", "label": "opensource.group.libraries" },
+  { "type": "dir", "name": "onixbyte-toolbox" },
+  { "type": "dir", "name": "regions4j" },
+  { "type": "dir", "name": "captcha" },
+  { "type": "dir", "name": "calendar-toolbox" },
+  { "type": "section-header", "label": "opensource.group.applications" },
+  { "type": "dir", "name": "helix" },
+  { "type": "dir", "name": "delta-force-guide" },
+  { "type": "section-header", "label": "opensource.group.tools" },
+  { "type": "dir", "name": "claude-code-skill-manager" },
+  { "type": "dir", "name": "vite-plugins" }
+]