Add DOM to glossary and links.

Author: stevepiercy

docs/backend/upgrading/version-specific-migration/p4x-to-p5x-upgrade.md

@@ -366,7 +366,7 @@ Here the two dependencies are jQuery and the pattern registry.
 We will not get into the pattern registry, as it is off-topic for this discussion.
 It is basically a registry of JavaScript components.
 
-The necessity for using it here is with AJAX calls and binding new DOM elements dynamically added to the page.
+The necessity for using it here is with AJAX calls and binding new {term}`DOM` elements dynamically added to the page.
 
 Additionally, above this `require` call, we provide some backward compatible code that you can inspect.
 

docs/glossary.md

@@ -216,7 +216,7 @@ SSR
 server-side rendering
     When a web browser or other HTTP client sends a request, the HTML markup for the page is created on the server, which sends a response consisting of HTML markup back to the client.
 
-    In Volto, SSR returns HTML markup that closely matches the final DOM structure of the React components used to render that page, but it is not the complete page.
+    In Volto, SSR returns HTML markup that closely matches the final {term}`DOM` structure of the React components used to render that page, but it is not the complete page.
     After the client loads the initial response, then the {term}`hydration` mechanism performs additional rendering on the client side, populating the DOM with additional HTML markup.
 
     In Classic UI, SSR returns the complete page back to the client in the response.
@@ -225,6 +225,12 @@ server-side rendering
     SSR enables a developer to customize a website per request and per user.
     In addition, SSR can improve performance and search engine optimization (SEO) for a website.
 
+DOM
+Document Object Model
+    The Document Object Model (DOM) is a programming interface for web documents.
+    It represents the page so that programs can change the document structure, style, and content.
+    The DOM represents the document as nodes and objects; that way, programming languages, such as JavaScript and React, can interact with the page.
+
 SPA
 single page application
     A type of JavaScript application that aims to provide a better user experience by avoiding unnecessary reloading of the browser page, instead using {term}`AJAX` to load backend information.
@@ -263,7 +269,7 @@ Yarn
     [Yarn](https://classic.yarnpkg.com/) is a JavaScript package manager.
 
 Hydration
-    After loading an HTML page generated with {term}`SSR` in the browser, React can populate the existing DOM elements, and recreate and attach their coresponding components.
+    After loading an HTML page generated with {term}`SSR` in the browser, React can populate the existing {term}`DOM` elements, and recreate and attach their coresponding components.
 
 JSX
     A dialect of JavaScript that resembles XML, it is transpiled by Babel to JavaScript functions.