docs: add VitePress documentation site

- Add docs directory with VitePress configuration
- Add GitHub Actions workflow for docs build and deploy
- Support bilingual (English/Chinese) documentation
- Include search, custom theme, and responsive design

docs/.vitepress/search/flexsearch.mjs

@@ -0,0 +1,25 @@
+export const FLEXSEARCH_INDEX_VERSION = 1
+
+export function flexsearchEncode(text) {
+  const normalized = String(text ?? '')
+    .toLowerCase()
+    .normalize('NFKC')
+
+  const tokens = normalized.match(
+    /[a-z0-9]+|[\u3040-\u30ff\u3400-\u4dbf\u4e00-\u9fff\uac00-\ud7af]/g
+  )
+
+  return tokens ?? []
+}
+
+export const FLEXSEARCH_OPTIONS = {
+  tokenize: 'forward',
+  resolution: 9,
+  cache: 100,
+  encode: flexsearchEncode
+}
+
+export function createFlexSearchIndex(FlexSearch) {
+  return new FlexSearch.Index(FLEXSEARCH_OPTIONS)
+}
+