ヘルプを Wiki から分離 (#424) (#430)

画面上でのテストは完了．
移行にす早さが求められるので，準備ができたらマージする．

Reviewed-on: #430
Co-authored-by: miteruzo <miteruzo@naver.com>

frontend/src/mdx-components.tsx

@@ -1,4 +1,52 @@
+import { useMemo } from 'react'
+import { useLocation } from 'react-router-dom'
+
+import PrefetchLink from '@/components/PrefetchLink'
+
 import type { MDXComponents } from 'mdx/types'
+import type { ComponentProps } from 'react'
 
 
-export const useMDXComponents = (): MDXComponents => ({ })
+const MDXLink = (
+    { href, children, ...props }: ComponentProps<'a'>,
+) => {
+  const location = useLocation ()
+  const url = useMemo (() => {
+    try
+    {
+      const base = new URL (location.pathname + location.search, window.location.origin)
+      return href ? new URL (href, base) : null
+    }
+    catch
+    {
+      return null
+    }
+  }, [href, location.pathname, location.search])
+  const internal = url != null
+      && url.origin === window.location.origin
+      && (url.protocol === 'http:' || url.protocol === 'https:')
+  const anchor = href?.startsWith ('#')
+      || (url?.pathname === location.pathname
+	  && url.search === location.search && Boolean (url.hash))
+  const external = url != null && !(internal)
+      && (url.protocol === 'http:' || url.protocol === 'https:')
+
+  if (internal && !(anchor) && props.download == null)
+  {
+    return (
+	<PrefetchLink to={url.pathname + url.search + url.hash} {...props}>
+	  {children}
+	</PrefetchLink>)
+  }
+
+  return (
+      <a href={href}
+	 {...props}
+	 target={external ? props.target ?? '_blank' : props.target}
+	 rel={props.rel ?? 'noopener noreferrer'}>
+	{children}
+      </a>)
+}
+
+
+export const useMDXComponents = (): MDXComponents => ({ a: MDXLink })