Docs: Hyphens are better than underscores for search engines

Google prefers hyphens when returning search results. Underscores are not read, and so a title like package_registry reads like packageregistry.

See their guidance here: https://support.google.com/webmasters/answer/76329?hl=en

This issue is to determine if or how we could ever rename our files to have hyphens instead of underscores.

Maybe we could at minimum update the guidance for new topics and folders to use hyphens? (Here is that guidance.)