Update colors of images to fall within WCAG guidelines

[curtisbarnard]

I've found the following images which need to also be updated to a contrast ratio of 7:1 or above:

• codec_history_timeline.png
• h264_bitstream_macro_diagram.png
• nal_units.png

Since I already did issue #130 I have the colors and templates set up so I can fix this as well. It might take a little longer as I need to find time over the next week.

[leandromoreira]

I'd be happy @curtisbarnard thank you so much :)