Option to setup and share a Secret URL for "Coming Soon" site

### What

I would like to have an option to setup and share a Secret URL from the site settings so users can send the secret URL to anyone on the web to see the site live while it is still in the **Coming Soon** state.

### Why

This will help the users to share the site without needing to invite people to the site as users. Plus, the visitors will not have to create an account to view the site making the whole process much more straightforward.

### How

We could add an option to show the in the **Privacy** settings.

Start in the site settings Privacy settings entry point and trace how access to a Coming Soon site is currently enforced. Define the secret URL's creation, sharing, and revocation behavior, then verify that anonymous visitors can view only the intended Coming Soon site without an account.

Written by the indexing model from the issue text.