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.

[skrawcz]

@alstondsouza1 thanks for putting this up. sorry for the slow review. mind resolving the conflict?

[alstondsouza1]

Thanks for reviewing! I've merged the latest main and resolved the conflicts, preserving the upstream updates in the relocated FastAPI and OpenLineage documentation pages. I also updated the new materialization link to the relocated OpenLineage page.

Verified all 161 documentation navigation targets and ran git diff --check. A full Sphinx build wasn't run locally because Sphinx isn't installed. GitHub now reports the branch as mergeable. Ready for another review.