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/testing/resources.adoc

@@ -8,7 +8,7 @@ See the following resources for more information about testing:
 * https://testng.org/[TestNG]: A testing framework inspired by JUnit with added support
   for test groups, data-driven testing, distributed testing, and other features. Supported
   in the xref:testing/testcontext-framework.adoc[Spring TestContext Framework]
-* https://assertj.github.io/doc/[AssertJ]: "Fluent assertions for Java",
+* {assertj-docs}[AssertJ]: "Fluent assertions for Java",
   including support for Java 8 lambdas, streams, and numerous other features.
 * https://en.wikipedia.org/wiki/Mock_Object[Mock Objects]: Article in Wikipedia.
 * http://www.mockobjects.com/[MockObjects.com]: Web site dedicated to mock objects, a
@@ -24,7 +24,7 @@ See the following resources for more information about testing:
 * https://www.dbunit.org/[DbUnit]: JUnit extension (also usable with Ant and Maven) that
   is targeted at database-driven projects and, among other things, puts your database into
   a known state between test runs.
-* https://www.testcontainers.org/[Testcontainers]: Java library that supports JUnit
+* {testcontainers-site}[Testcontainers]: Java library that supports JUnit
   tests, providing lightweight, throwaway instances of common databases, Selenium web
   browsers, or anything else that can run in a Docker container.
 * https://sourceforge.net/projects/grinder/[The Grinder]: Java load testing framework.