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/blogs/_meta.json

@@ -0,0 +1,33 @@
+[
+  "index",
+  { "type": "section-header", "label": "insights.group.frontend" },
+  { "type": "file", "name": "ecmascript-2025-syntax-sugar" },
+  { "type": "file", "name": "frontend-development-standards" },
+  { "type": "file", "name": "frontend-tips-and-solutions" },
+  { "type": "file", "name": "font-size-conversion-table" },
+  { "type": "section-header", "label": "insights.group.backend" },
+  { "type": "file", "name": "java-development-standards" },
+  { "type": "file", "name": "java-development-cheatsheet" },
+  { "type": "file", "name": "mybatis-flex" },
+  { "type": "file", "name": "postgresql-zhparser-fuzzy-search" },
+  { "type": "file", "name": "use-json-in-mysql" },
+  { "type": "section-header", "label": "insights.group.devops" },
+  { "type": "file", "name": "docker-deployment-standards" },
+  { "type": "file", "name": "gitlab-ops" },
+  { "type": "file", "name": "setup-ldap-service" },
+  { "type": "file", "name": "minio-admin-guide" },
+  { "type": "section-header", "label": "insights.group.practice" },
+  { "type": "file", "name": "general-development-standards" },
+  { "type": "file", "name": "google-code-review-standards" },
+  { "type": "file", "name": "version-control-and-code-review" },
+  { "type": "file", "name": "git-cheatsheet" },
+  { "type": "section-header", "label": "insights.group.tooling" },
+  { "type": "file", "name": "fix-idea-application-yaml-schema-validation" },
+  { "type": "file", "name": "windows-driver-backup-restore" },
+  { "type": "file", "name": "fix-macos-monterey-sleep-wake" },
+  { "type": "file", "name": "macos-private-dns-hostname" },
+  { "type": "file", "name": "quartile-method" },
+  { "type": "file", "name": "use-of-gravatar" },
+  { "type": "file", "name": "email-like-a-boss" },
+  { "type": "file", "name": "cooking-seasoning-guide" }
+]