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/zh-hans/blogs/index.md

@@ -1,10 +1,10 @@
 ---
-title: 博客
+title: 技术洞察
 ---
 
-# 博客
+# 技术洞察
 
-欢迎来到 OnixByte 博客，这里汇集了日常工程实践中沉淀的实用指南、速查手册和开发规范。
+这里汇集了日常工程实践中沉淀的实用指南、速查手册和开发规范。
 
 ## 话题分类