Add documentation on how to get this GitHub action to work from scratch

# Task Description

It would be helpful to add some documentation on how to get this GitHub action to work from scratch. It may look straight-forward to someone who's done it a thousand times, but -- in reality -- it currently isn't.

For example, I am assuming that you need to create an application under `Computer` / `Workers & Pages`. However, this doesn't let you do so. If I select `Create Application` / `Connect GitHub` , it will ask me to approve some repository. I obviously am here, because I don't want this to be handled on Cloudflare's side, but -- rather -- via the GitHub Action. I also don't want it to create a "Hello, World!"-like application because that would deploy some demo app and I already have an `mkdocs` site which I'd like to publish.

So, like... some basic steps to get you set up before you can actually use this GitHub action, would be really helpful. Things like:
* How to create the application
* How to create the secrets / token properly
* How to secure the site with login credentials

# Task Relationships

* cloudflare/workers-sdk#9255

Start by reviewing the repository's existing setup documentation and the related cloudflare/workers-sdk#9255 issue. Document the prerequisites and step-by-step flow for creating the Cloudflare application, configuring secrets or tokens, deploying an existing mkdocs site, and securing it with login credentials.

Written by the indexing model from the issue text.