Add more descriptive content to commands documentation

Basically updating the files under doc/cli_commands

One example is this documentation page, that explains more about the command than before.

List of updated files:

• actev_design_chunks.md
• actev_exec.md
• actev_experiment_cleanup.md
• actev_experiment_init.md
• actev_get_system.md
• actev_merge_chunks.md
• actev_post_process_chunk.md
• actev_pre_process_chunk.md
• actev_process_chunk.md
• actev_reset_chunk.md
• actev_status.md
• actev_system_setup.md
• actev_validate_execution.md
• actev_validate_system.md