docs: fix sidebar structure and duplicate page headings

Chronicle renders the frontmatter title as the page heading, so remove
the duplicate H1 from every page. Add meta.json to each section folder
to set sidebar labels and ordering (fumadocs folders are not ordered by
page frontmatter). Rename the concepts overview page title to Overview.

Author: ravisuhag

docs/.data/chronicle-search.sqlite

@@ -0,0 +1,4 @@
+{
+  "title": "Concepts",
+  "order": 4
+}