Add Donkey Pro to build_hardware.md

Update install_software.md

m: Fix Markdown for MkDocs

Incorrect Directory in Software Installation Document

more robust and user-friendly calibration

Added Validation of user input for PWM calibration

Losslessly Optimized Images

remove non core parts

[ImgBot] Optimize images

*Total -- 16,047.74kb -> 10,816.81kb (32.6%)

/docs/assets/build_hardware/2b.PNG -- 228.28kb -> 117.31kb (48.61%)
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/docs/assets/build_hardware/donkey2.PNG -- 909.86kb -> 530.89kb (41.65%)
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/docs/assets/build_hardware/assemble camera.jpg -- 81.97kb -> 78.86kb (3.8%)
/docs/assets/drive_UI.png -- 98.45kb -> 98.13kb (0.32%)

Update build_hardware.md

Print message when setting PWM fails.

This prevents the car from completely stopping if there is a weak connection
between the host and the peripheral motor board. Otherwise, a single failed
set_pwm call will halt the host.

fix Hyperlink error

* Simple exception handling to avoid failing

set JOYSTICK_THROTTLE_AXIS = 'ry' in config.py, so that 'Right Stick' key on
the controller can be used to move the car forth and back by setting throttle

1. ` and ' typo.
2. Mkdocs needs an emtpy line before the itemization.

Update index.md

fix typo