Make headers on react-server.io linkable

[roblg]

I wanted to link to "Use Custom Express Middleware" on https://react-server.io/docs/guides/react-server-cli, but I can't. Would be great if we could!

[drewpc]

I was thinking the same thing. Something like this maybe?

[roblg]

I'm in favor of whatever works. :) (Not super familiar with how the site is structured yet.)