Fix README CLI-flag docs and markdown syntax

[nu-jliu]

Summary

Corrects documentation defects across the repo's READMEs that were found by auditing every documented CLI command/flag against the actual arg parsers, plus two markdown-rendering/link fixes. Also makes one small source change so the viser tool rejects an arm choice that produces a degenerate (dead-IK) session. Several of these were hard breaks — documented commands that fail outright (wrong flag name, wrong script path, broken Python import).

Changes

• devices/pi_setup.md: split two malformed single-line code fences (```bash ... ```) into proper multi-line blocks; the unclosed fence made the rest of the doc render as one code block
• examples/bimanual_lead_follower/README.md: repoint broken link from the moved guide ../../doc/set_persist_id_socket_can.md to ../../docs/guides/set-persistent-can-ids.md
• i2rt/flow_base/README.md: fix speed/accel source link anchor #L500-L501 -> #L742-L743 (the actual max_vel/max_accel definitions)
• README.md: fix CLI flag --gripper_type -> --gripper (motor_chain_robot defines --gripper); fix Python API import to i2rt.robots.get_robot and pass GripperType.LINEAR_4310 (a string raised AttributeError via gripper_type.get_xml_path()); use the canonical tyro flag --bilateral-kp
• examples/control_with_mujoco/README.md: add no_arm to --arm and flexible_4310 to --gripper (choices are built from the full enums); correct --site (default is auto-detected from gripper, not grasp_site)
• examples/control_with_viser/control_with_viser.py: exclude ArmType.NO_ARM from --arm choices; the viewer's IK and arm-joint modes are degenerate with no arm (_n_arm=0)
• examples/control_with_viser/README.md: add flexible_4310 to --gripper; keep --arm at the four supported variants to match the restricted choices
• examples/single_motor_position_pd_control/README.md: correct script path to examples/single_motor_position_pd_control/single_motor_position_pd_control.py (was a non-existent i2rt/scripts/single_motor_pd_pos_control.py)

Test Plan

• Markdown render (GitHub): open devices/pi_setup.md in the PR diff and confirm sections 4 & 5 render as real code blocks (not one giant block); confirm the bimanual link and the flow_base #L742-L743 anchor both resolve.
• viser choices (no hardware): python examples/control_with_viser/control_with_viser.py --help shows --arm {yam,yam_pro,yam_ultra,big_yam} and flexible_4310 in --gripper; python examples/control_with_viser/control_with_viser.py --arm no_arm --sim exits with error: argument --arm: invalid choice: 'no_arm'.
• mujoco choices (no hardware): python examples/control_with_mujoco/control_with_mujoco.py --help shows no_arm in --arm and flexible_4310 in --gripper.
• minimum_gello flag (no hardware): python examples/minimum_gello/minimum_gello.py --help lists --bilateral-kp (and accepts it).
• single_motor path: examples/single_motor_position_pd_control/single_motor_position_pd_control.py exists and defines --channel, --motor_id, --kd.
• Python API (full env): python -c "from i2rt.robots.get_robot import get_yam_robot; from i2rt.robots.utils import GripperType; get_yam_robot(channel='can0', gripper_type=GripperType.LINEAR_4310, sim=True)" runs with no ImportError/AttributeError.
• Hardware (optional): on a station with CAN, python i2rt/robots/motor_chain_robot.py --channel can0 --gripper linear_4310 starts in zero-gravity mode.

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