Add a glossary to the documentation (#647)

Based on
https://github.com/frequenz-floss/frequenz-sdk-python/wiki/Glossary,
with a lot of improvements and additions.

Fixes #142.

Author: llucax

docs/SUMMARY.md

@@ -1,3 +1,4 @@
 * [Home](index.md)
+* [Introduction](intro/)
 * [API Reference](reference/)
 * [Contributing](CONTRIBUTING.md)