Spring Extract recurring asciidoc links to attributes

This commit extracts 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 - Java API Documentation (Java 17 version) - Kotlin documentation (Kotlinlang.org version) - the Spring Boot reference guide (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.

Closes gh-26864

Comment From: jhoeller

Looks good to me as well! Let's do this for 6.1.1 right away.

Comment From: simonbasle

the only thing in the deleted asciidoc folder that could be of later use is the (as of now unsupported in antora toolchain) anchor-rewrite.properties file