Add javafmtFormatJavadoc setting

[mkurz]

Adds a first-class sbt setting for controlling google-java-format's Javadoc formatting behavior:

• javafmtFormatJavadoc

This follows the same direction as the recent CLI-backed settings such as:

• javafmtSortImports
• javafmtRemoveUnusedImports
• javafmtReflowLongStrings

Instead of requiring users to go through javafmtOptions, the common sbt-facing configuration is now exposed directly as a dedicated setting:

ThisBuild / javafmtFormatJavadoc := false

Why this change?

Since the plugin now runs google-java-format in a forked JVM via its CLI, the natural public API is the set of CLI-backed formatter toggles exposed as normal sbt settings. That makes configuration more explicit and easier to discover in the README, and
reduces reliance on the upstream JavaFormatterOptions object as the primary user-facing API.

This PR also updates the README to clarify the current state of javafmtOptions:

• it is still available for compatibility
• the preferred configuration path is through the dedicated javafmt... settings
• JavaFormatterOptions.reorderModifiers() currently has no effect in this plugin, because the released google-java-format CLI used here does not yet support a corresponding --skip-reordering-modifiers flag. See:
  • Support reorderModifiers = false via google-java-format CLI #266
  • Add CLI flag to disable modifier reordering google/google-java-format#1373

Finally, this adds a scripted test that verifies javafmtFormatJavadoc := false preserves Javadoc content formatting while still applying normal code formatting around it.