feat(docs): collapse sidebar sections by default

Activates fumadocs' built-in collapse on every section. Only the
section containing the current page expands; others stay collapsed
until the user clicks them. Foundation for the typographic sidebar
pass in the next commit.

docs/content/docs/guides/meta.json

@@ -0,0 +1,5 @@
+{
+  "title": "Guides",
+  "defaultOpen": false,
+  "pages": ["building-context", "writing-context", "serving-agents"]
+}