Add concepts back into glossary

[strickvl]

Pre-requisites

Please ensure you have done the following:

• I have read the CONTRIBUTING.md document.
• If my change requires a change to the documentation, I have updated the documentation accordingly.
• I have added tests to cover my changes.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Describe changes

• Added concepts back into glossary, sorted alphabetically.

(I removed some code examples to make them concise. I also didn't spend too much time thinking through what concepts are missing. I figure we'll go through these properly when we redo the website / messaging etc post-retreat).

[htahir1]

Nice! we should add services to this when its done?

[strickvl]

@htahir1 yes I feel like there will be a few changes / additions to this that will come. but this is good enough for the short-term. A lot of these concepts were written by me when I didn't really know / understand much so they can be made much more concise.

[htahir1]

@htahir1 Then its LGTM!