feat: provide a pathname:// protocol to escape SPA

[samestep]

Resolves #1685 by providing the same solution Docusaurus has.

The new example in docs/guide/asset-handling.md exercises the change to src/node/markdown/plugins/link.ts. To exercise the change to src/client/theme-default/support/utils.ts, apply this diff and run pnpm run docs:

diff --git a/docs/.vitepress/config.ts b/docs/.vitepress/config.ts
index 53a7068..3e56e60 100644
--- a/docs/.vitepress/config.ts
+++ b/docs/.vitepress/config.ts
@@ -53,7 +53,9 @@ export default defineConfig({
 })
 
 function nav() {
+  const link = 'pathname:///pure.html'
   return [
+    { text: link, link },
     { text: 'Guide', link: '/guide/what-is-vitepress', activeMatch: '/guide/' },
     { text: 'Configs', link: '/config/introduction', activeMatch: '/config/' },
     {