Use consistent capitalization in User Guide Sections

[swzCuroverse]

https://www.commonwl.org/user_guide/

Some are capitalized like titles and others like sentences (but aren't sentences)

I suggest we capitalize them all like titles.

i.e. Using containers --> Using Containers

[kinow]

Mea culpa. And +1. Will submit a PR soon (< couple days?) if no one beats me to it.