Generate module-style Javadoc

[cpovirk]

To avoid:

warning: The code being documented uses modules but the packages defined in https://jspecify.dev/docs/api/ are in the unnamed module.

See #484 (comment).

The big question would be whether generating module-style Javadoc makes it harder for non-module users to link to us.

[scordio]

As a workaround for projects that use -Werror during Javadoc generation, the --link-modularity-mismatch=(warn|info) option introduced in Java 18 can be used to specify how external documentation with wrong modularity should be reported.

For example, the following Maven configuration works without failures:

<plugin>
  <groupId>org.apache.maven.plugins</groupId>
  <artifactId>maven-javadoc-plugin</artifactId>
  <version>3.11.2</version>
  <configuration>
    <additionalOptions>--link-modularity-mismatch=info</additionalOptions>
    <failOnWarnings>true</failOnWarnings>
    <links>
      <link>https://jspecify.dev/docs/api/</link>
    </links>
    ...
  </configuration>
</plugin>

See also JDK-8274639.