Add docs outline with Antora skeleton

docs/modules/guides/nav.adoc

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+.xref:index.adoc["How-to" Guides]
+* xref:page-1.adoc[Example 1]

docs/modules/guides/pages/index.adoc

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+= "How-to" Guides
+
+This page is under construction.
+
+== Table of contents
+
+** Obtain an access token using a specific grant_type:
+*** `authorization_code`
+*** `client_credentials`
+*** `refresh_token`
+** Customize form based login
+** Authenticate a user with two-factor authentication
+** Customize the user consent page
+** Authenticate using OpenID Connect 1.0 authorization_code flow
+** Customize the OpenID Connect 1.0 UserInfo response
+** Authenticate using social login, e.g. Google
+** Authenticate a user in a Single Page Application with PKCE
+** Customize client authentication for specific authentication methods
+** Handle errors and customize the OAuth 2.0 Error response
+** Authorize an access token containing custom authorities, e.g. roles, groups, permissions, etc.
+*** Customize the headers / claims in a JWT
+** Deny access for a revoked JWT access token
+*** Introspect / revoke an access token
+** Provide a JWK source backed by a key rotation strategy
+** Implement the core services with JPA:
+*** `RegisteredClientRepository`
+*** `OAuth2AuthorizationService`
+*** `OAuth2AuthorizationConsentService`

docs/modules/guides/pages/page-1.adoc

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+= Example 1