doc: add undici to glossary

PR-URL: https://github.com/nodejs/node/pull/43327
Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Akhil Marsonya <akhil.marsonya27@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

glossary.md

@@ -18,3 +18,5 @@ You may also need to check <https://chromium.googlesource.com/chromiumos/docs/+/
   primarily administrated by Matt Godbolt.
 * primordials: Pristine built-ins that are not affected by prototype pollution
   and tampering with built-ins.
+* undici: An alternative HTTP client used in Node.js. See more details
+  on the [undici repository](https://github.com/nodejs/undici).