Migrate ci to nimby

[treeform]

• Replace the old CI setup with actions/checkout@v5, treeform/setup-nim-action@v6, and Nimby dependency installation from the GitHub workspace parent.
• Run the VMath default, object-based, array-based, and JavaScript test modes as separate YAML steps.
• Remove the non-default memory manager CI step and rename the test entrypoint to tests/tests.nim.
• Convert docs publishing from nimble to Nimby plus the required nim doc command.