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/index.md

@@ -1,11 +1,11 @@
 ---
-title: Blog
+title: Insights
 ---
 
-# Blog
+# Insights
 
-Welcome to the OnixByte blog — a collection of practical guides, cheatsheets, and standards
-distilled from daily engineering work.
+Practical guides, cheatsheets, and standards distilled from daily engineering
+work.
 
 ## Topics