Update links to Spotless' new documentation layout
Update links to Spotless' new documentation layout

Also included the Spotless maven plugin (2+ years old, but not well marketed)

Fixes #509

COPYBARA_INTEGRATE_REVIEW=https://github.com/google/google-java-format/pull/509 from nedtwigg:patch-2 546c758cc5be91141118cc1b9c560b082ea191cd
PiperOrigin-RevId: 322832707

sormuras/google-java-format,sormuras/google-java-format,sormuras/google-java-format