Add documentation for file_uploader transformer

[Copilot]

The file_uploader transformer lacked documentation, making it difficult for users to discover and use this functionality for uploading files from local to remote nodes.

Changes

• Added documentation section in docs/run_test/transformers.rst following existing transformer documentation patterns
• Documented required parameters: source (local path), destination (remote path), files (list of file names)
• Documented output variable: uploaded_files
• Included YAML usage example:

transformer:
  - type: file_uploader
    phase: environment_connected
    source: "/local/path/to/files"
    destination: "/remote/path/to/upload"
    files:
      - "file1.txt"
      - "file2.sh"
      - "config.yaml"

The transformer creates the destination directory if it doesn't exist and uploads each specified file from the source directory. It should be used in the environment_connected phase when the environment is already deployed and connected, so no explicit connection configuration is needed.

Original prompt

add doc for file_uploader transformer

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[adityagesh]

LGTM