minor #17048 Linking to ux.symfony.com (weaverryan)

This PR was submitted for the 6.1 branch but it was merged into the 5.4 branch instead.

Discussion
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Linking to ux.symfony.com

Actually, more links are needed:

A) `_ux_libraries.rst.inc` should include a demo link for each
B) The actual `index.rst` file of each bundle (over on symfony/ux) should include a demo link.

But... I'm about to leave for a week, so I at least wanted to get this started - it's already overdue :).

Commits
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d54600c Linking to ux.symfony.com

Author: javiereguiluz

frontend/ux.rst

@@ -4,6 +4,10 @@
 The Symfony UX Initiative & Packages
 ====================================
 
+.. tip::
+
+    Check out live demos of Symfony UX at `https://ux.symfony.com`_!
+
 Symfony UX is an initiative and set of libraries to seamlessly
 integrate JavaScript tools into your application. For example,
 want to render a chart with `Chart.js`_? Use `UX Chart.js`_ to build the
@@ -148,3 +152,4 @@ the Node package that is responsible for a lot of the magic.
 .. _`@symfony/stimulus-bridge`: https://github.com/symfony/stimulus-bridge
 .. _`stimulus-use`: https://stimulus-use.github.io/stimulus-use
 .. _`stimulus-components`: https://stimulus-components.netlify.app/
+.. _`https://ux.symfony.com`: https://ux.symfony.com