Add canvas previews, external extension links, and release notes showcase (#1987)

* Add extension thumbnails and preview assets

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Add clickable extension image preview modal

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Address PR review feedback

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Update website/src/styles/global.css

* Add preview assets for canvas extensions

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Update canvas extension preview images

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Delete extensions/backlog-swipe-triage/assets/swipe-canvas-triage.png

* Support external canvas extensions and add Coffilot

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Add homepage link to GitHub repository

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Add release notes showcase canvas extension

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Enhance release notes canvas sourcing and layout

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

---------

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Co-authored-by: Aaron Powell <me@aaron-powell.com>

extensions/external.json

@@ -0,0 +1,10 @@
+[
+  {
+    "id": "coffilot",
+    "name": "Coffilot",
+    "description": "Java-focused Copilot canvas extension from jdubois.",
+    "installUrl": "https://github.com/jdubois/coffilot",
+    "sourceUrl": "https://github.com/jdubois/coffilot",
+    "imagePath": "extensions/external-assets/coffilot-preview.png"
+  }
+]