Highlight major features of cwltool

[mr-c]

• [x] Add cwltool installation instructions
• [x] Allow cwltool specifics to appear in example outputs. Colors, etc..
• [x] Add a Note about running a CWL worfklow/CLT using a generic CWL runner (cwl-runner my-description.cwl my-inputs.{yaml,json})
• [ ] Highlight major features of cwltool
  • [x] cwltool --validate
  • [x] cwltool --make-template
  • [x] cwltool --cachedir
  • [ ] cwltool --parallel
  • [ ] cwltool --singularity & CWL_SINGULARITY_CACHE

[kinow]

I've checked the items already done in the PR's for Sphinx and new structure.

For the other ones I will prepare the changes in https://github.com/common-workflow-language/user_guide/pull/246 :+1: