Auto-reload sidebar when things change

Our sidebar generator currently doesn't work well with the hot reload feature.

This should be fixable by also writing a plugin, which can modify the watcher, as is done here: https://github.com/QC2168/vite-plugin-vitepress-auto-sidebar/blob/dc0ae3ddfb68fb20c4f2464e2c8f61233d0262cc/src/index.ts#L107