Fix: Correction of the siteRoot path (#17297)

- For the first issue, when a user set ./docs to the default value of
ScalaSettings, the code only took a value that was not the default
`siteRoot.nonDefault`. To fix this, I put a getOrElse to try and get the
default value.


- For the second problem, I noticed when I cloned the code that the user
was calling layouts that did not exist. Be careful with this as it can
indeed produce an error. Solving this problem was enough to remove the
non-existent layout calls.

Fixes: #15306
[Cherry-picked 4e4552e]

scaladoc/src/dotty/tools/scaladoc/Scaladoc.scala

@@ -202,7 +202,7 @@ object Scaladoc:
         classpath.get,
         bootclasspath.get,
         destFile,
-        siteRoot.nonDefault,
+        Option(siteRoot.withDefault(siteRoot.default)),
         projectVersion.nonDefault,
         projectLogo.nonDefault,
         projectFooter.nonDefault,