Add explanation of how the React scheduler finds the next unit of work

[mudassir-jmi]

Description

This PR adds documentation explaining how React's scheduler algorithm determines which units of work to process next. The content covers the priority system, work queue management, and the yielding mechanism that keeps React applications responsive.

Details

• Added clear explanation of how different priorities are assigned to work items
• Included simplified code examples to illustrate the scheduler's decision-making process
• Made the content accessible to developers who may not be familiar with React internals
• Filled the gap mentioned in the "Future sections" note about scheduler algorithms

I'd appreciate any feedback on technical accuracy or suggestions for improvement.

[sid2213]

Please don't use Chatgpt.