feat(theme): allow overriding last updated time in doc footer from frontmatter (#2848)

docs/reference/frontmatter-config.md

@@ -163,10 +163,10 @@ The levels of header in the outline to display for the page. It's same as [confi
 
 ### lastUpdated
 
-- Type: `boolean`
+- Type: `boolean | Date`
 - Default: `true`
 
-Whether to display [last updated](./default-theme-last-updated) text in the footer of the current page.
+Whether to display [last updated](./default-theme-last-updated) text in the footer of the current page. If a datetime is specified, it will be displayed instead of the last git modified timestamp.
 
 ```yaml
 ---

src/client/theme-default/components/VPDocFooterLastUpdated.vue

@@ -2,9 +2,11 @@
 import { ref, computed, watchEffect, onMounted } from 'vue'
 import { useData } from '../composables/data'
 
-const { theme, page } = useData()
+const { theme, page, frontmatter } = useData()
 
-const date = computed(() => new Date(page.value.lastUpdated!))
+const date = computed(
+  () => new Date(frontmatter.value.lastUpdated ?? page.value.lastUpdated)
+)
 const isoDatetime = computed(() => date.value.toISOString())
 const datetime = ref('')