Updated url(..) with path(..) in documentation

[saichenko]

Description

According to the official Django documentation:

url() deprecated since version 3.1:
Alias of django.urls.re_path() for backwards compatibility.

Changes

I updated url()'s with path()'s in index.md, authentication.md and generic-views.md

[lovelydinosaur]

Closed via #7512, which includes this work.
Thank you!