Extract recurring asciidoc links to attributes, cleanup old doc files

This commit extract spring-related links and recurring external links
into asciidoctor attributes to be used by the Antora toolchain.

It notably homogenizes links to:
 - IETF RFCs
 - Java Community Process JSRs
 - the Java API Documentation (on the Java 17 version)
 - Kotlin documentations (on the Kotlinlang.org version)
 - the Spring Boot reference guide (on the `html` version)

This commit also reworks most link attributes to follow a
Project-Category-Misc syntax. For example, `spring-boot-docs` rather
than `docs-spring-boot`.

Finally, it makes an effort to clean up remainders from the previous
documentation toolchain, namely the `docs/asciidoc` folder and 
`modules/ROOT/pages/attributes.adoc` file.

Closes gh-26864
Closes gh-31619

framework-docs/modules/ROOT/pages/web/websocket/stomp/authentication-token-based.adoc

@@ -1,7 +1,7 @@
 [[websocket-stomp-authentication-token-based]]
 = Token Authentication
 
-https://github.com/spring-projects/spring-security-oauth[Spring Security OAuth]
+{spring-github-org}/spring-security-oauth[Spring Security OAuth]
 provides support for token based security, including JSON Web Token (JWT).
 You can use this as the authentication mechanism in Web applications,
 including STOMP over WebSocket interactions, as described in the previous
@@ -11,13 +11,13 @@ At the same time, cookie-based sessions are not always the best fit (for example
 in applications that do not maintain a server-side session or in
 mobile applications where it is common to use headers for authentication).
 
-The https://tools.ietf.org/html/rfc6455#section-10.5[WebSocket protocol, RFC 6455]
+The {rfc-site}/rfc6455#section-10.5[WebSocket protocol, RFC 6455]
 "doesn't prescribe any particular way that servers can authenticate clients during
 the WebSocket handshake." In practice, however, browser clients can use only standard
 authentication headers (that is, basic HTTP authentication) or cookies and cannot (for example)
 provide custom headers. Likewise, the SockJS JavaScript client does not provide
 a way to send HTTP headers with SockJS transport requests. See
-https://github.com/sockjs/sockjs-client/issues/196[sockjs-client issue 196].
+{sockjs-client}/issues/196[sockjs-client issue 196].
 Instead, it does allow sending query parameters that you can use to send a token,
 but that has its own drawbacks (for example, the token may be inadvertently
 logged with the URL in server logs).

framework-docs/modules/ROOT/pages/web/websocket/stomp/authorization.adoc

@@ -6,7 +6,7 @@ Spring Security provides
 {docs-spring-security}/servlet/integrations/websocket.html#websocket-authorization[WebSocket sub-protocol authorization]
 that uses a `ChannelInterceptor` to authorize messages based on the user header in them.
 Also, Spring Session provides
-https://docs.spring.io/spring-session/reference/web-socket.html[WebSocket integration]
+{docs-spring-session}/web-socket.html[WebSocket integration]
 that ensures the user's HTTP session does not expire while the WebSocket session is still active.
 
 

framework-docs/modules/ROOT/pages/web/websocket/stomp/enable.adoc

@@ -91,7 +91,7 @@ and xref:web/websocket/stomp/authentication.adoc[Authentication] for more inform
 
 For more example code see:
 
-* https://spring.io/guides/gs/messaging-stomp-websocket/[Using WebSocket to build an
+* {spring-site-guides}/gs/messaging-stomp-websocket/[Using WebSocket to build an
 interactive web application] -- a getting started guide.
 * https://github.com/rstoyanchev/spring-websocket-portfolio[Stock Portfolio] -- a sample
 application.

framework-docs/modules/ROOT/pages/web/websocket/stomp/handle-broker-relay.adoc

@@ -59,7 +59,7 @@ The following example shows the XML configuration equivalent of the preceding ex
 ----
 
 The STOMP broker relay in the preceding configuration is a Spring
-{api-spring-framework}/messaging/MessageHandler.html[`MessageHandler`]
+{spring-framework-api}/messaging/MessageHandler.html[`MessageHandler`]
 that handles messages by forwarding them to an external message broker.
 To do so, it establishes TCP connections to the broker, forwards all messages to it,
 and then forwards all messages received from the broker to clients through their

framework-docs/modules/ROOT/pages/web/websocket/stomp/message-flow.adoc

@@ -7,18 +7,18 @@ connected clients. This section describes the flow of messages on the server sid
 The `spring-messaging` module contains foundational support for messaging applications
 that originated in https://spring.io/spring-integration[Spring Integration] and was
 later extracted and incorporated into the Spring Framework for broader use across many
-https://spring.io/projects[Spring projects] and application scenarios.
+{spring-site-projects}[Spring projects] and application scenarios.
 The following list briefly describes a few of the available messaging abstractions:
 
-* {api-spring-framework}/messaging/Message.html[Message]:
+* {spring-framework-api}/messaging/Message.html[Message]:
 Simple representation for a message, including headers and payload.
-* {api-spring-framework}/messaging/MessageHandler.html[MessageHandler]:
+* {spring-framework-api}/messaging/MessageHandler.html[MessageHandler]:
 Contract for handling a message.
-* {api-spring-framework}/messaging/MessageChannel.html[MessageChannel]:
+* {spring-framework-api}/messaging/MessageChannel.html[MessageChannel]:
 Contract for sending a message that enables loose coupling between producers and consumers.
-* {api-spring-framework}/messaging/SubscribableChannel.html[SubscribableChannel]:
+* {spring-framework-api}/messaging/SubscribableChannel.html[SubscribableChannel]:
 `MessageChannel` with `MessageHandler` subscribers.
-* {api-spring-framework}/messaging/support/ExecutorSubscribableChannel.html[ExecutorSubscribableChannel]:
+* {spring-framework-api}/messaging/support/ExecutorSubscribableChannel.html[ExecutorSubscribableChannel]:
 `SubscribableChannel` that uses an `Executor` for delivering messages.
 
 Both the Java configuration (that is, `@EnableWebSocketMessageBroker`) and the XML namespace configuration