Add NeuroPawn Knight IMU Board Support and Gain Configuration

[neuropawn-admin]

This PR introduces support for the NeuroPawn Knight IMU board and adds gain configuration capabilities for Knight boards.

Key Changes:

1. New Board Support:

• Added KnightIMU class (src/board_controller/neuropawn/knightimu.h and .cpp) to handle data acquisition and parsing for the Knight IMU board (Board ID: 66, NEUROPAWN_KNIGHT_BOARD_IMU)
• Updated BrainFlowBoards definitions in src/board_controller/brainflow_boards.cpp to include the new board constants and default settings
• Updated BoardController factory in src/board_controller/board_controller.cpp to initialize the KnightIMU board

2. Gain Configuration

• Updated Knight and KnightIMU classes to parse gain settings from the params.other_info JSON string (e.g., '{"gain": 6}')
• Supported gain values: 1, 2, 3, 4, 6, 8, 12 (default is 12)
• Fixed a scaling bug related to ADC handling in knight.cpp

3. Documentation

• Updated docs/SupportedBoards.rst to document usage of the Knight IMU board.
• Refined initialization examples to demonstrate how to configure gain and select between Knight and Knight IMU boards using BoardIds