Headings in templates & guide should be sentence case

[ncbaratta]

A few years ago a decision was made in CCS that all headings should be sentence case. We should follow that guideline in this guide and in the templates we provide.

[emmurphy1]

Please review the following PR where I fixed the headings and subheadings:
#189

[emmurphy1]

@adahms no need to add this to the decision register.