docs(authentication): Improve auth docs to match current allauth best practices and syntax

[jinnatar]

• Allauth has changed the OIDC provider to use the APPS syntax instead of SERVERS
• The link to all providers was dead.
• Allauth recommends that a provider should be configured through only one method, and weird issues can arise when there's a conflicting provider_id via ENV and Django Admin.
• PKCE is starting to be very common and required, added that to the example. (Separate PR to improve openid_connect docs in django-allauth for that.)

[vabene1111]

perfect. thank you very much!