doc: add glossary to module reference documentation

These terms will be defined throughout the document, and more terms
will be added. After drafting a few sections, it's clear that a
glossary will be useful. There are enough terms that it would be
overwhelming at the beginning.

Also, add anchors for each heading and add a couple more headings.

Updates #33637

Change-Id: I0017064f0b1e5e656dd280018ca0379484345df1
Reviewed-on: https://go-review.googlesource.com/c/go/+/206478


Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Tyler Bui-Palsulich <tbp@google.com>