Move the documentation package to its own subproject (#722)

This makes it easier to later migrate the package to Java 11. If we move
and migrate in a single PR, because of the portion of the contents that
s changed, git will have trouble recognizing that some files are
renamed *and* modified and treat them as distinct files, making code
review difficult.

settings.gradle

@@ -30,6 +30,7 @@ rootProject.name = 'nomulus'
 include 'common'
 include 'core'
 include 'db'
+include 'docs'
 include 'integration'
 include 'networking'
 include 'processor'