Merge branch 'docs-topic-auth' into 'master'

Documentation topic: Authentication

Closes #30744

See merge request !10608

doc/topics/authentication/index.md

+# Authentication
+
+This page gathers all the resources for the topic **Authentication** within GitLab.
+
+## GitLab users
+
+- [SSH](../../ssh/README.md)
+- [Two-Factor Authentication (2FA)](../../user/profile/account/two_factor_authentication.md#two-factor-authentication)
+- **Articles:**
+  - [Support for Universal 2nd Factor Authentication - YubiKeys](https://about.gitlab.com/2016/06/22/gitlab-adds-support-for-u2f/)
+  - [Security Webcast with Yubico](https://about.gitlab.com/2016/08/31/gitlab-and-yubico-security-webcast/)
+- **Integrations:**
+  - [GitLab as OAuth2 authentication service provider](../../integration/oauth_provider.md#introduction-to-oauth)
+
+## GitLab administrators
+
+- [LDAP (Community Edition)](../../administration/auth/ldap.md)
+- [LDAP (Enterprise Edition)](https://docs.gitlab.com/ee/administration/auth/ldap-ee.md)
+- [Enforce Two-factor Authentication (2FA)](../../security/two_factor_authentication.md#enforce-two-factor-authentication-2fa)
+- **Articles:**
+  - [Feature Highlight: LDAP Integration](https://about.gitlab.com/2014/07/10/feature-highlight-ldap-sync/)
+  - [Debugging LDAP](https://about.gitlab.com/handbook/support/workflows/ldap/debugging_ldap.md)
+- **Integrations:**
+  - [OmniAuth](../../integration/omniauth.md)
+  - [Authentiq OmniAuth Provider](../../administration/auth/authentiq.md#authentiq-omniauth-provider)
+  - [Atlassian Crowd OmniAuth Provider](../../administration/auth/crowd.md)
+  - [CAS OmniAuth Provider](../../integration/cas.md)
+  - [SAML OmniAuth Provider](../../integration/saml.md)
+  - [Okta SSO provider](../../administration/auth/okta.md)
+  - [Kerberos integration (GitLab EE)](https://docs.gitlab.com/ee/integration/kerberos.md)
+
+## API
+
+- [OAuth 2 Tokens](../../api/README.md#oauth-2-tokens)
+- [Private Tokens](../../api/README.md#private-tokens)
+- [Impersonation tokens](../../api/README.md#impersonation-tokens)
+- [GitLab as an OAuth2 provider](../../api/oauth2.md#gitlab-as-an-oauth2-provider)
+- [GitLab Runner API - Authentication](../../api/ci/runners.md#authentication)
+
+## Third-party resources
+
+- [Kanboard Plugin GitLab Authentication](https://kanboard.net/plugin/gitlab-auth)
+- [Jenkins GitLab OAuth Plugin](https://wiki.jenkins-ci.org/display/JENKINS/GitLab+OAuth+Plugin)
+- [Setup Gitlab CE with Active Directory authentication](https://www.caseylabs.com/setup-gitlab-ce-with-active-directory-authentication/)
+- [How to customize GitLab to support OpenID authentication](http://eric.van-der-vlist.com/blog/2013/11/23/how-to-customize-gitlab-to-support-openid-authentication/)
+- [Openshift - Configuring Authentication and User Agent](https://docs.openshift.org/latest/install_config/configuring_authentication.html#GitLab)