Add glossary and clarification on opaque strings and identifiers

[kinow]

For common-workflow-language/common-workflow-language#914.

concepts.md is included by Workflow.yml and Process.yml, so the link should render OK. But I'll try to render it before marking as ready for review.

[kinow]

Huh? Did I merge it?

[kinow]

Ah. I rebased onto the wrong branch, and then GitHub considered my branch as merged and closed it 😅 sweaty palms for a few seconds. All sorted out now, nothing to see here 👷

[kinow]

Table of Contents, now with the 1.7 Glossary entry.

Content rendered.

What links look like:

Tested each link generated, and all of them worked with no issues 👍 Ready for review.

[kinow]

And Today-I-Learned how to build our website. And also that we use cwltool to build the CWL website. That's meta! 🤓

[mr-c]

Looks great, thanks!

[kinow]

ping @tetron :)