Feature: Add addtional `visibility` configuration option for hiding pages from search

[leadenhall]

We'd like an option to hide pages from search but still allow them to be found in the navigation view. There are occasions where pages are drafted but not polished enough for appearing in search but be explicitly available in a known area. Useful for ensuring that search results only include end-user facing and actionable documentation.

https://retype.com/configuration/page/#visibility