Restructure the packages in `spring-boot-docs` so that they mirror
the documentation sections. There are now three main packages:
`springbootfeatures`, `productionreadyfeatures` and `howto`. Each
of the main packages has a subpackage named after the section headings.

Example code now uses consistent `// tag::` names and imports are
applied using `[tag=*]` whenever possible.

Test snippets have been moved to `src/main/java` so that only a single
import attribute needs to be defined.

Closes gh-25089