Document "Overview" in reference manual

Closes gh-667

Author: Steve Riesenberg

docs/modules/ROOT/nav.adoc

@@ -1,5 +1,5 @@
-.xref:index.adoc[Reference Manual]
-* xref:overview.adoc[Overview]
+.Reference Manual
+* xref:index.adoc[Overview]
 * xref:getting-help.adoc[Getting Help]
 * xref:getting-started.adoc[Getting Started]
 * xref:configuration-model.adoc[Configuration Model]

docs/modules/ROOT/pages/index.adoc

@@ -1,3 +1,46 @@
-= Reference Manual
+= Overview
 
-This page is under construction.
+This site contains reference documentation and how-to guides for Spring Authorization Server.
+
+[[introducing-spring-authorization-server]]
+== Introducing Spring Authorization Server
+
+Spring Authorization Server is a framework that provides implementations of the OAuth 2.1 and OpenID Connect 1.0 specifications and other related specifications.
+It is built on top of https://spring.io/projects/spring-security[Spring Security] to provide a secure, light-weight, and customizable foundation for building identity providers and authorization server products.
+
+[[feature-list]]
+== Feature List
+
+Spring Authorization Server supports the following features:
+
+* The OAuth 2.1 Authorization Framework (https://datatracker.ietf.org/doc/html/draft-ietf-oauth-v2-1-05[Draft])
+** Authorization Grant
+*** https://datatracker.ietf.org/doc/html/draft-ietf-oauth-v2-1-05#section-4.1[Authorization Code]
+*** https://datatracker.ietf.org/doc/html/draft-ietf-oauth-v2-1-05#section-4.2[Client Credentials]
+*** https://datatracker.ietf.org/doc/html/draft-ietf-oauth-v2-1-05#section-4.3[Refresh Token]
+** Access Token Format
+*** Self-contained (JWT)
+*** Reference (Opaque)
+** https://datatracker.ietf.org/doc/html/draft-ietf-oauth-v2-1-05#section-2.4[Client Authentication]
+*** HTTP Basic
+*** HTTP POST
+*** JSON Web Token (JWT) Profile for OAuth 2.0 Client Authentication (https://tools.ietf.org/html/rfc7523[RFC 7523])
+**** `private_key_jwt`
+**** `client_secret_jwt`
+** User Consent
+*** https://datatracker.ietf.org/doc/html/draft-ietf-oauth-v2-1-05#section-4.1[Authorization Code Grant]
+* Proof Key for Code Exchange by OAuth Public Clients (PKCE) (https://tools.ietf.org/html/rfc7636[RFC 7636])
+* OAuth 2.0 Token Revocation (https://tools.ietf.org/html/rfc7009[RFC 7009])
+* OAuth 2.0 Token Introspection (https://tools.ietf.org/html/rfc7662[RFC 7662])
+* OAuth 2.0 Authorization Server Metadata (https://tools.ietf.org/html/rfc8414[RFC 8414])
+* JSON Web Token (JWT) (https://tools.ietf.org/html/rfc7519[RFC 7519])
+* JSON Web Signature (JWS) (https://tools.ietf.org/html/rfc7515[RFC 7515])
+* JSON Web Key (JWK) (https://tools.ietf.org/html/rfc7517[RFC 7517])
+* OpenID Connect Core 1.0 (https://openid.net/specs/openid-connect-core-1_0.html[spec])
+** https://openid.net/specs/openid-connect-core-1_0.html#CodeFlowAuth[Authorization Code Flow]
+** https://openid.net/specs/openid-connect-core-1_0.html#UserInfo[UserInfo Endpoint]
+* OpenID Connect Discovery 1.0 (https://openid.net/specs/openid-connect-discovery-1_0.html[spec])
+** https://openid.net/specs/openid-connect-discovery-1_0.html#ProviderConfig[Provider Configuration Endpoint]
+* OpenID Connect Dynamic Client Registration 1.0 (https://openid.net/specs/openid-connect-registration-1_0.html[spec])
+** https://openid.net/specs/openid-connect-registration-1_0.html#ClientRegistration[Client Registration Endpoint]
+** https://openid.net/specs/openid-connect-registration-1_0.html#ClientConfigurationEndpoint[Client Configuration Endpoint]