Remove Project (Volto) from Glossary, because it is not used in the documentation.

Add term `add-on` to install-from-packages.md and rewrite its definition in Glossary.

Author: stevepiercy

docs/glossary.md

@@ -154,11 +154,17 @@ record
     A Rapido app is able to store data as records.
     Records are basic dictionaries.
 
-Project (Volto)
-    The product of running the package `@plone/generator-volto`, resulting in a customizable instance of Volto.
+Add-on
+    An add-on in Plone extends its functionality.
+    It is code that is released as a package to make it easier to install.
 
-Add-on (Volto)
-    A JavaScript package that integrates with Volto's configuration registry and is able to enhance, extend, and customize it.
+    In Volto, an add-on is a JavaScript package.
+
+    In Plone core, an add-on is a Python package.
+
+    -   [Plone core add-ons](https://github.com/collective/awesome-plone#readme)
+    -   [Volto add-ons](https://github.com/collective/awesome-volto#readme)
+    -   [Add-ons tagged with the trove classifier `Framework :: Plone` on PyPI](https://pypi.org/search/?c=Framework+%3A%3A+Plone)
 
 Add-on configuration loader (Volto)
     A function with signature `config => config`.

docs/install/install-from-packages.md

@@ -21,7 +21,7 @@ When you want full control over development or deployment, installing Plone from
 
 The hardware requirements below give a rough estimate of the minimum hardware setup needed for a Plone server.
 
-Add-on products and caching solutions may increase RAM requirements.
+{term}`Add-on` products and caching solutions may increase RAM requirements.
 
 A single Plone installation is able to run many Plone sites.
 You may host multiple Plone sites on the same server.