docs: add Learning Hub article on installing and using plugins

- New article covering plugin concepts, finding/installing/managing plugins,
  marketplace browsing, and how this repo serves as a plugin marketplace
- Add 'installing-and-using-plugins' to fundamentalsOrder in index.astro
  and ArticleLayout.astro
- Add Plugin entry to terminology glossary

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

website/src/layouts/ArticleLayout.astro

@@ -25,6 +25,7 @@ const fundamentalsOrder = [
   'understanding-mcp-servers',
   'automating-with-hooks',
   'using-copilot-coding-agent',
+  'installing-and-using-plugins',
   'before-after-customization-examples',
 ];