Docs still reference deprecated Netlify Identity

[3aluw]

Some parts of the Decap CMS documentation still instruct users to set up Netlify Identity for authentication. However, Netlify Identity has been deprecated. example

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.