Design tweaks to project metrics charts

[cjyabraham]

As per G:

• Make color swatches look on/off buttons
• Dates are hard to read; reduce the size and frequency of dates; get rid of day number
• Make the grey lines and text darker and bolder
• Add chart page to brand guidelines

[cjyabraham]

I've made some improvements here: https://www.cncf.io/project-metrics/