Generate markdown-formatted javadoc

[jwharm]

Gtk API documentation (gi-docgen) is in markdown format. To generate clean-looking Javadoc, java-gi runs a series of regular expressions to translate markdown formatting to HTML. This works well enough in many cases, but some formatting, like tables, are not translated.

With JEP 467, OpenJDK 23 introduces markdown-formatted Javadoc. This would be a natural fit for java-gi. However, there are two disadvantages:

• JavaPoet doesn't support newer JDK features, and has been in maintenance mode with no new releases planned. So it will probably need to be forked to generate the new Javadoc syntax.
• This will raise the minimum required Java version to OpenJDK 23. All users will have to upgrade.