Prevent linter / lintUnused warning

This warning now no longer happens on sbt startup:
```
[warn] there's a key that's not used by any other settings/tasks:
[warn]
[warn] * solrs / Paradox / sourceDirectory
[warn]   +- /home/magro/proj/solrs/build.sbt:72
[warn]
[warn] note: a setting might still be used by a command; to exclude a key from this `lintUnused` check
[warn] either append it to `Global / excludeLintKeys` or call .withRank(KeyRanks.Invisible) on the key
```

build.sbt

@@ -71,6 +71,9 @@ Test / fork := true
 enablePlugins(ParadoxSitePlugin)
 Paradox / sourceDirectory := sourceDirectory.value / "main" / "paradox"
 
+// prevent linter warning "there's a key that's not used by any other settings/tasks"
+Global / excludeLintKeys += Paradox / sourceDirectory
+
 enablePlugins(GhpagesPlugin)
 git.remoteRepo := scmInfo.value.get.connection