Feature request: optional 'noindex' option to stop indexing by Google

[JasperHG90]

Hi,

Thanks for this wonderful package!

Most of my projects live in private repositories until they are released. However, I like to add documentation and share that with a select few people using GitHub pages. I don't really mind that the documentation is public, but it would be great to be able to tell e.g. Google not to index the page. This can be achieved by adding the the following to index.html:

<meta name="robots" value="noindex" />

It would be nice if users could toggle this on/off using a setting in _pkgdown.yml.

Thanks!