Add built-in search plugin to config example

[infotexture]

When I copied the config example here, the search box disappeared from my output.

After a bit of research, I found this in the Material theme docs:

The built-in search plugin integrates seamlessly with Material for MkDocs, adding multilingual client-side search with lunr and lunr-languages. It's enabled by default, but must be re-added to mkdocs.yml when other plugins are used

To prevent confusion, I've added that line to the config example.