Improve CSRF Documentation

[israel-oye]

This update extends the documentation regarding CSRF protection in the library. Specifically, it provides additional insights into the 'csrf' input name configuration for Non-FlaskForms in preventing the common "CSRF token is missing." Users can now better understand the implication of the WTF_CSRF_FIELD_NAME config.

Additionally, a new example has been included in the JavaScript section that demonstrates how to send the 'csrf_token' as a Jinja2 variable (to the backend) with the Fetch API using a Vanilla JavaScript example. This example shows the proper implementation of CSRF protection when working with the Fetch API.