The `javadoc-baseurl` asciidoctor attribute is now externalized
(i.e. not included directly in the document anymore).
This allows to properly render javadoc links in single pages, whereas
those URLs were previoulsy only supported in the single page version.
The reference documentation is now organized in asciidoc bookparts.
Bookparts sections can be also put in separate files, when their
size and theme are important enough to justify that.
Documentation Layout:
```
index
|-- overview
|
|-- whats-new
|
|-- core
| |-- core-beans
| |-- core-aop
|-- testing
|
|-- data-access
|
|-- web
| |-- web-mvc
| |-- web-view
|-- integration
|
|-- appendix
|-- appx-spring-tld
|-- appx-spring-form-tld
```
Supersedes and closes#641
Issue: SPR-12309
