Improve format of navigation to home (#50665)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/50665

Changelog: [internal]

Just a stylistic change of the link back to the documentation home.

It also changes the link in the template to make sure it's incorrect so people are forced to edit it when creating new docs.

Reviewed By: lenaic

Differential Revision: D72866823

fbshipit-source-id: fa7be286f996049b3f06e2defa5a8c486d63091b