Add a section to docs for variables and symbols

We need a single place in the documentation where one can find all important variable names, their meanings, and name changes from one era to the next. This should probably be a table with the variable name or symbol, a very brief definition, and links to where the variable or symbol first appears.

Maybe this could be part of the Notation section (which already exists) or maybe it should go in a table in the appendix.

Start by reviewing the existing Notation section and the appendix to determine where a consolidated reference belongs. Inventory the important variable names and symbols, their brief meanings, historical name changes, and links to where each first appears. Done means the documentation has a single, navigable table covering the relevant notation.

Written by the indexing model from the issue text.