Add support for --cross in the doc sub-command

[Gedochao]

Adds support for --cross generating documentation with doc.

//> using scala 2.13 3.3
package lib

/** Example class for cross-built documentation. */
class Example {
  /** Returns a greeting. */
  def greet: String = "Hello"
}

scala-cli --power doc --cross . -o doc-out
# Wrote Scaladoc to doc-out/2.13.18
# Wrote Scaladoc to doc-out/3.3.7

Checklist

• tested the solution locally and it works
• ran the code formatter (scala-cli fmt .)
• ran scalafix (./mill -i __.fix)
• ran reference docs auto-generation (./mill -i 'generate-reference-doc[]'.run)

How much have your relied on LLM-based tools in this contribution?

extensively, Cursor + Claude

How was the solution tested?

automated tests included

Additional notes

• Run all cross builds when --cross is passed #3808 added the same for run
• Support --cross with the package sub-command #4171 added the same for package