[docs] move building client only routes into its own guide

Description

~The guide on client only routes and user authentication is an important enough guide to be its own page.~

~Another issue came up while working on #15979, linking to the subheading for client only routes in the “Building Apps with Gatsby” page means if we change the subheading’s title the links in other places will break, making that link very fragile, and giving us reason to turn it into its own page.~

Action Steps

• ~move the section from client only routes and user authentication into it’s own file~
• ~once #15979 is merged, we’ll need to update the link in the list of the tags in the tags-docs.yaml to point to the guide in the new location~
• update the links in other places of the docs that referred to the old doc to point to the new page

[sedat]

@marcysutton @Jhony0311 totally fine for me as I haven’t had a change because of work and was thinking of finishing it this weekend.

[marcysutton]

@Jhony0311 that sounds cool to me! It might be smart to wait until https://github.com/gatsbyjs/gatsby/pull/16309 is merged, even though #15979 is in already. Hopefully should be done today.