Add `homepage` example without the full domain

Created by: camjackson

Consider 3 different possibilities for where an app will be deployed:

1. Deploying to just a single domain and path (e.g. always example.com/my-app)
2. Deploying to any arbitrary domain and path (e.g. example.com/my-app but also random.com/blah)
3. Deploying to multiple domains but always the same path (e.g. example.com/my-app and test.example.com/my-app)

Scenario 1 is well-documented at "Deployment - Building for relative paths":

  "homepage": "http://mywebsite.com/relativepath",

Scenario 2 is well-documented at Deployment - Serving the Same Build from Different Paths, including the caveat that it breaks client-side routing:

  "homepage": ".",

Scenario 3 does not have a documented solution, despite it being the most common one in my experience. As far as I can tell, the following configuration works perfectly well:

  "homepage": "/my-app",

But after much searching I haven't seen anywhere on the internet that mentions the ability to specify homepage with just a path and no domain.

This PR adds that example to the docs. Here's how it looks: