Modularize Asciidoc [SPR-12309]

[spring-projects-issues]

Sébastien Deleuze opened SPR-12309 and commented

As contributed by Arjen Poutsma in his pull request #641, the reference documentation should be re-modularized into separate files, just like it was in the DocBook days.

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Reference URL: #641

Referenced from: commits 7407223, 0baf228

[spring-projects-issues]

Brian Clozel commented

Fixed with #751