Update introduction and terminology.

[nathan-at-least]

This updates introduction.md and terminology.md only, so other pages may be inconsistent:

Changes to Introduction

• Ensure introduction links to any defined term.
• Remove the word "block" entirely from the introduction.

Changes to Terminology

• Remove all definitions for which there are not explicit reference links. (Note, other text may still use some of the removed terms, which should be cleaned up in future PRs.)
• Remove "term categories" and just sort them alphabetically.
• Streamline the terminology intro text.

Related Tickets

• Closes #59

[nathan-at-least]

I committed all of @daira's suggested improvements.