Add more examples for app.get_relative_url in public docs

Although app.get_relative_url is mostly used for Dash Enterprise, I feel that including it in more examples (or at least have a mention about it) would be beneficial for building apps that will just work on DE. A note should be added in the following pages:

• [ ] Multi-Page Apps and URL Support
• [ ] Devtools
• [ ] dcc.Link

At the moment it's only available on this page, which is not the first thing that I would think of when I look for the correct way to access a certain server route.

• [x] Just added app.get_relative_path to the new reference page