Refactor Asciidoctor attributes in reference docs

This commit reorganizes the asciidoctor attributes for the reference
documentation. Instead of being contributed partially by the build and
individual documents, attributes are now shared in common files that are
included in top sections.

framework-docs/src/docs/asciidoc/web/webflux-cors.adoc

@@ -1,6 +1,6 @@
 [[webflux-cors]]
 = CORS
-:doc-spring-security: {doc-root}/spring-security/reference
+:doc-spring-security: {docs-site}/spring-security/reference
 [.small]#<<web.adoc#mvc-cors, Web MVC>>#
 
 Spring WebFlux lets you handle CORS (Cross-Origin Resource Sharing). This section

framework-docs/src/docs/asciidoc/web/webflux.adoc

@@ -1,7 +1,7 @@
 [[webflux]]
 :chapter: webflux
 = Spring WebFlux
-:doc-spring-security: {doc-root}/spring-security/reference
+:doc-spring-security: {docs-site}/spring-security/reference
 
 The original web framework included in the Spring Framework, Spring Web MVC, was
 purpose-built for the Servlet API and Servlet containers. The reactive-stack web framework,

framework-docs/src/docs/asciidoc/web/webmvc-cors.adoc

@@ -1,6 +1,6 @@
 [[mvc-cors]]
 = CORS
-:doc-spring-security: {doc-root}/spring-security/reference
+:doc-spring-security: {docs-site}/spring-security/reference
 [.small]#<<web-reactive.adoc#webflux-cors, WebFlux>>#
 
 Spring MVC lets you handle CORS (Cross-Origin Resource Sharing). This section

framework-docs/src/docs/asciidoc/web/webmvc.adoc

@@ -1,7 +1,7 @@
 [[mvc]]
 :chapter: mvc
 = Spring Web MVC
-:doc-spring-security: {doc-root}/spring-security/reference
+:doc-spring-security: {docs-site}/spring-security/reference
 
 Spring Web MVC is the original web framework built on the Servlet API and has been included
 in the Spring Framework from the very beginning. The formal name, "`Spring Web MVC,`"

framework-docs/src/docs/asciidoc/web/websocket.adoc

@@ -1,6 +1,6 @@
 [[websocket]]
 = WebSockets
-:doc-spring-security: {doc-root}/spring-security/reference
+:doc-spring-security: {docs-site}/spring-security/reference
 [.small]#<<web-reactive.adoc#webflux-websocket, WebFlux>>#
 
 This part of the reference documentation covers support for Servlet stack, WebSocket