Add `springDocsUrl` Asciidoctor attribute to point to the Spring Framework reference docs

[odrotbohm]

To be able to easily adapt to a changed URI structure of Spring Frameworks reference docs, we should add an Asciidoctor attribute that points to the base URI of their docs so that it can be referenced like:

link:{springDocsUrl}/core.html#context-functionality-events[Spring application events]