Improve documentation structure for plugins and cookbook

[alstondsouza1]

Summary

This PR addresses #1043 by improving the organization of the documentation.

Changes

• Split the old Integrations section into Plugins and Cookbook sections.
• Added a Plugin API Reference section.
• Kept lifecycle interfaces under the Lifecycle API section.
• Moved concrete adapters such as ProgressBar and SlackNotifier into the plugin reference.
• Updated navigation and internal links that referenced the old paths.
• Updated example README links.

Testing

• Verified moved pages and toctree targets exist.
• Checked for stale references to old documentation paths.
• Ran git diff --check.
• Full Sphinx documentation build will be performed through CI.

Closes #1043.