docs: add docs about lastUpdated (#1731)

docs/guide/theme-last-updated.md

@@ -1,3 +1,25 @@
 # Last Updated
 
-Documentation coming soon...
+The update time of the last content will be displayed in the lower right corner of the page.
+To enable it, add `lastUpdated` options to your config.
+
+## Page Configuration
+
+Add `lastUpdated` options to your config.
+
+```js
+export default {
+  lastUpdated: true
+}
+```
+
+## Frontmatter Configuration
+
+If you would like to hide the last update text, set false to the `lastUpdated` option.
+
+```yaml
+---
+lastUpdated: false
+---
+```
+