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

[gillkyle]

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 fix(blog): blog tag audit and overhaul #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]

I can take on this!

[marcysutton]

One other thing I would add is that we should still mention client-only routes on the Building Apps with Gatsby page, and link to the new standalone page for more information.

[marcysutton]

Also, sounds great @sedat!

[Jhony0311]

@marcysutton @sedat Is okay if I take this? Already created the new page and waiting on #15979 to be merged. Is okay to open a PR?

[marcysutton]

@Jhony0311 that sounds cool to me! It might be smart to wait until #16309 is merged, even though #15979 is in already. Hopefully should be done today.

[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.

[Jhony0311]

@marcysutton Should I update every reference on other files as well? I did a quick search and I see this references:

• docs/docs/routing.md
• docs/blog/2018-11-07-gatsby-for-apps/index.md
• packages/gatsby-plugin-netlify/README.md

I imagine the plugin and the blog post don't apply but routing seems like a yes to me.

Also could this updates be a redirect maybe?

[gillkyle]

Oh just merged the PR (thanks @Jhony0311!) to make those a new guide, but if those links:

• docs/docs/routing.md
• docs/blog/2018-11-07-gatsby-for-apps/index.md
• packages/gatsby-plugin-netlify/README.md

are broken we should update them yes! If either of you two: @sedat or @Jhony0311 are interested a PR to update those would be awesome, if not I'm sure someone else would be happy to pick it up!

[Jhony0311]

Happy to take that, I can go ahead and update that. Should I do all three @gillkyle ? Just want to make sure that's okay.

[gillkyle]

@Jhony0311 yeah it makes sense to have them all in one PR thanks!