This commit moves Maven plugin content from several sections in the
main Spring Boot reference documentation to the plugin-specific
documentation.
Fixes gh-19165
With the introduction of Gradle we lost the list of version properties
that were previously in the spring-boot-dependencies POM and were also
linked inside the documentation. This commit introduces an appendix
section in the docs and links the appropriate places to the new section
to restore discoverability.
See gh-19898
