Add documentation for program blocks

[abhishekck31]

This PR adds documentation explaining program blocks in Music Blocks.

The documentation describes what program blocks are, their role in controlling
execution flow and organization, how they differ from sound blocks, and how
common program blocks such as Start, Action, and Loop blocks are used in
projects. It also includes best practices for organizing and structuring music
using program blocks.

I explored the existing documentation and codebase to understand how program
blocks function in Music Blocks and wrote the documentation accordingly. AI
tools were used to assist in understanding the code, but the documentation
content and structure were written manually.

Closes #3844

[github-actions]

❌ Some Jest tests failed. Please check the logs and fix the issues before merging.

Failed Tests:

SaveInterface.test.js

[walterbender]

It might make sense to link to this documentation from https://github.com/sugarlabs/musicblocks/blob/master/Docs/documentation/README.md