Support Markdown documentation comments in spring-boot-configuration-processor

https://openjdk.org/jeps/467, currently targeted at Java 23, will add support for writing documentation comments using Markdown. The JEP describes some additions to the API that may affect our annotation processor:

We introduce a new type of tree node, RawTextTree, which contains uninterpreted text, together with a new tree-node kind, DocTree.Kind.MARKDOWN, which indicates Markdown content in a RawTextTree. We add corresponding new visitRawText methods to DocTreeVisitor and its subtypes, DocTreeScanner and DocTreePathScanner.

While some exploratory work may be worthwhile, this is largely blocked until the changes to JavaDoc have been finalised.

Start by reading JEP 467 and the spring-boot-configuration-processor implementation to identify where documentation comments are parsed. Check whether the JavaDoc API changes have been finalized before proceeding; the issue is done when the processor supports the finalized Markdown comment representation and its affected behavior is verified.