Docs still reference deprecated Netlify Identity

Some parts of the Decap CMS documentation still instruct users to set up **Netlify Identity** for authentication. However, Netlify Identity has been **deprecated**. [example](https://decapcms.org/docs/nuxt/)

This could confuse new users who follow the docs and end up using unsupported auth methods.

**Request:**

* Update the docs to direct users toward the **currently supported authentication methods**.
* Add a **deprecation notice** wherever Netlify Identity is mentioned, to prevent new setups from relying on it.

**Impact:**
Ensuring the docs are up to date will save users time and frustration, and help them adopt modern, supported authentication workflows.