images: Create /authentication_backends directory for icons.

Creates `static/images/authentication_backends` directory for icons
of backend authentication methods, which are used on the log-in page.

And updates the example documentation in the API `/server_settings`
endpoint.

zerver/openapi/zulip.yaml

@@ -13321,14 +13321,14 @@ paths:
                             {
                               "name": "google",
                               "display_name": "Google",
-                              "display_icon": "/static/images/landing-page/logos/googl_e-icon.png",
+                              "display_icon": "/static/images/authentication_backends/googl_e-icon.png",
                               "login_url": "/accounts/login/social/google",
                               "signup_url": "/accounts/register/social/google",
                             },
                             {
                               "name": "github",
                               "display_name": "GitHub",
-                              "display_icon": "/static/images/landing-page/logos/github-icon.png",
+                              "display_icon": "/static/images/authentication_backends/github-icon.png",
                               "login_url": "/accounts/login/social/github",
                               "signup_url": "/accounts/register/social/github",
                             },

zproject/backends.py

@@ -1851,7 +1851,7 @@ class GitHubAuthBackend(SocialAuthMixin, GithubOAuth2):
     name = "github"
     auth_backend_name = "GitHub"
     sort_order = 100
-    display_icon = "/static/images/landing-page/logos/github-icon.png"
+    display_icon = "/static/images/authentication_backends/github-icon.png"
 
     def get_all_associated_email_objects(self, *args: Any, **kwargs: Any) -> List[Dict[str, Any]]:
         access_token = kwargs["response"]["access_token"]
@@ -1946,15 +1946,15 @@ class AzureADAuthBackend(SocialAuthMixin, AzureADOAuth2):
     sort_order = 50
     name = "azuread-oauth2"
     auth_backend_name = "AzureAD"
-    display_icon = "/static/images/landing-page/logos/azuread-icon.png"
+    display_icon = "/static/images/authentication_backends/azuread-icon.png"
 
 
 @external_auth_method
 class GitLabAuthBackend(SocialAuthMixin, GitLabOAuth2):
     sort_order = 75
     name = "gitlab"
     auth_backend_name = "GitLab"
-    display_icon = "/static/images/landing-page/logos/gitlab-icon.png"
+    display_icon = "/static/images/authentication_backends/gitlab-icon.png"
 
     # Note: GitLab as of early 2020 supports having multiple email
     # addresses connected with a GitLab account, and we could access
@@ -1970,7 +1970,7 @@ class GoogleAuthBackend(SocialAuthMixin, GoogleOAuth2):
     sort_order = 150
     auth_backend_name = "Google"
     name = "google"
-    display_icon = "/static/images/landing-page/logos/googl_e-icon.png"
+    display_icon = "/static/images/authentication_backends/googl_e-icon.png"
 
     def get_verified_emails(self, *args: Any, **kwargs: Any) -> List[str]:
         verified_emails: List[str] = []
@@ -2001,7 +2001,7 @@ class AppleAuthBackend(SocialAuthMixin, AppleIdAuth):
     sort_order = 10
     name = "apple"
     auth_backend_name = "Apple"
-    display_icon = "/static/images/landing-page/logos/apple-icon.png"
+    display_icon = "/static/images/authentication_backends/apple-icon.png"
 
     # Apple only sends `name` in its response the first time a user
     # tries to sign up, so we won't have it in consecutive attempts.