Document source build path for Cromwell forks

[Copilot]

Description

This addresses the gap behind “I want to build the cromwell fork” by adding explicit local build instructions in the repository root docs. The change makes the fork’s build path discoverable without digging through CI config.

• Build prerequisites
  • Added required runtime/tooling versions: Java 17 and sbt 1.8.2.
• Canonical build command
  • Added the supported assembly command for producing the server artifact.
• Artifact location
  • Documented the expected output JAR path for quick verification/consumption.

sbt server/assembly
# -> server/target/scala-2.13/cromwell-<version>.jar

Release Notes Confirmation

CHANGELOG.md

• I updated CHANGELOG.md in this PR
• I assert that this change shouldn't be included in CHANGELOG.md because it doesn't impact community users

Terra Release Notes

• I added a suggested release notes entry in this Jira ticket
• I assert that this change doesn't need Jira release notes because it doesn't impact Terra users

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