Add documentation for sign-in application setting

changelogs/unreleased/35547-add-documentation-for-sign-in-application-setting.yml

+---
+title: Add documentation for sign-in application setting
+merge_request: 19561
+author: Horatiu Eugen Vlad
+type: added

doc/user/admin_area/settings/index.md

@@ -14,6 +14,7 @@ include:
 - [Continuous Integration and Deployment](continuous_integration.md)
 - [Email](email.md)
 - [Sign up restrictions](sign_up_restrictions.md)
+- [Sign in restrictions](sign_in_restrictions.md)
 - [Terms](terms.md)
 - [Third party offers](third_party_offers.md)
 - [Usage statistics](usage_statistics.md)

doc/user/admin_area/settings/sign_in_restrictions.md

+---
+type: reference
+---
+
+# Sign-in restrictions **(CORE ONLY)**
+
+You can use sign-in restrictions to limit the authentication with password
+for web interface and Git over HTTP(S), two-factor authentication enforcing, as well as
+as configuring the home page URL and after sign-out path.
+
+## Password authentication enabled
+
+You can restrict the password authentication for web interface and Git over HTTP(S):
+
+- **Web interface**: When this feature is disabled, an [external authentication provider](../../../administration/auth/README.md) must be used.
+- **Git over HTTP(S)**: When this feature is disabled, a [Personal Access Token](../../profile/personal_access_tokens.md) must be used to authenticate.
+
+## Two-factor authentication
+
+When this feature enabled, all users will have to use the [two-factor authentication](../../profile/account/two_factor_authentication.md).
+
+Once the two-factor authentication is configured as mandatory, the users will be allowed
+to skip forced configuration of two-factor authentication for the configurable grace
+period in hours.
+
+![Two-factor grace period](img/two_factor_grace_period.png)
+
+## Sign-in information
+
+All users that are not logged-in will be redirected to the page represented by the configured
+"Home page URL" if value is not empty.
+
+All users will be redirect to the page represented by the configured "After sign out path"
+after sign out if value is not empty.
+
+If a "Sign in text" in Markdown format is provided, then every user will be presented with
+this message after logging-in.
+
+## Settings
+
+To access this feature:
+
+1. Navigate to the **Settings > General** in the Admin area.
+1. Expand the **Sign-in restrictions** section.
+
+<!-- ## Troubleshooting
+
+Include any troubleshooting steps that you can foresee. If you know beforehand what issues
+one might have when setting this up, or when something is changed, or on upgrading, it's
+important to describe those, too. Think of things that may go wrong and include them here.
+This is important to minimize requests for support, and to avoid doc comments with
+questions that you know someone might ask.
+
+Each scenario can be a third-level heading, e.g. `### Getting error message X`.
+If you have none to add when creating a doc, leave this section in place
+but commented out to help encourage others to add to it in the future. -->