Add a summary for Introduction to state management

[segunolalive]

Add a summary for Introduction to state management.

Note

• We prefer video summaries to be 2 or 3 paragraphs long. Summaries could be longer if absolutely necessary. You should add a bullet list of things the viewer will learn at the end.
• Summaries should be written in markdown.
• Summaries should be saved in the src/session-summaries folder.
• Summary files should be named with the session slug e.g The slug for the Design Systems Part 2 video is design-systems-part-2, so the summary file for that session should be named design-systems-part-2.md.
• Select the Session Summary PR Template when making a Pull Request.