docs: update Learning Hub for Copilot CLI v1.0.61-v1.0.64 (#2119)

- understanding-mcp-servers: add .github/mcp.json to config discovery table (v1.0.61)
- installing-and-using-plugins: add extensions as a plugin component type (v1.0.62)
- copilot-configuration-basics: add /settings, /worktree, and /diagnose commands (v1.0.61, v1.0.64); note per-model token totals in /usage (v1.0.64)

Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

website/src/content/docs/learning-hub/installing-and-using-plugins.md

@@ -3,7 +3,7 @@ title: 'Installing and Using Plugins'
 description: 'Learn how to find, install, and manage plugins that extend GitHub Copilot CLI with reusable agents, skills, hooks, and integrations.'
 authors:
   - GitHub Copilot Learning Hub Team
-lastUpdated: 2026-04-27
+lastUpdated: 2026-06-24
 estimatedReadingTime: '8 minutes'
 tags:
   - plugins
@@ -33,6 +33,7 @@ A plugin bundles one or more of the following components:
 | **Hooks** | Event handlers that intercept agent behavior | `hooks.json` or `hooks/` |
 | **MCP Servers** | Model Context Protocol integrations for external tools | `.mcp.json` or `.github/mcp.json` |
 | **LSP Servers** | Language Server Protocol integrations | `lsp.json` or `.github/lsp.json` |
+| **Extensions** | IDE extensions installable via the plugin marketplace (v1.0.62+) | `extensions/` |
 
 A plugin might include all of these or just one — for example, a plugin could provide a single specialized agent, or an entire development toolkit with multiple agents, skills, hooks, and MCP server configurations working together.