docs(gallery): add documentation for new gallery-item component

[brandyscarney]

⚠️ Must remove dev builds from StackBlitz and demos before merging. ⚠️

This PR adds the new Gallery Item component and updates the existing Gallery playgrounds and documentation.

Gallery
Gallery Item

Note: you will have to update any StackBlitz examples that use images to the following to see them load:

<ion-gallery-item>
  <img 
-    src="https://ionicframework.com/docs/img/demos/gallery/01.png" 
+    src="https://ionic-docs-git-fw-7301-ionic1.vercel.app/docs/img/demos/gallery/01.png"
    alt="Image 1"
  />
</ion-gallery-item>

[vercel]

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Project	Deployment	Actions	Updated (UTC)
ionic-docs	Ready	Preview, Comment	Jun 9, 2026 10:03pm

[brandyscarney]

Note: this file has conflicts with major-9.0 that I am purposely not resolving right now because it uses an Ionic Angular version that supports Angular 21 but my dev build is off a different branch and it will not work with Angular 21.

[ShaneK]

Looks good to me, awesome work!