Rewrite the auth example to be based on the increment example

[leighmcculloch]

What

Rewrite the auth example to be based on the increment example

Why

So that all the smaller examples are similar and tell a story for how to add a feature to a contract. This was a suggestion from @tomerweller. I've tried it on a few examples and it seems to work pretty well I think.

[leighmcculloch]

@sisuresh Wdyt of this? If you think this is good I'd like to also update the advanced auth example so it does auth for incrementing a value, but as features on the incremental example. I'd probably remove the admin feature and just make it so there's a user who is authenticated. Here's what the docs will look like once updated:

• https://github.com/stellar/soroban-docs/pull/178
• https://soroban-docs-pr178.previews.kube001.services.stellar-ops.com/docs/examples/auth

[sisuresh]

Yeah I think this makes sense, and the docs for this look great. Thanks!

[leighmcculloch]

Thanks for reviewing. I'll do the same to the advanced auth example too.